[vs18.0] Add null check for environmentVariableProperties

[github-actions]

Backport of #12594 to vs18.0

Fixes https://dev.azure.com/devdiv/DevDiv/_workitems/edit/2586915/

Context

the recent change ac5fef4#diff-40f77147ccc6867c10324ec4ddf8110ede2e43499d939b73ef83c7f5febb7a75 doesn't have null check on an attempt to get property from _environmentVariableProperties collection that can be null.

Changes Made

Handle possible null ref exception.

More details

In the referenced ticket you can find a dump and check the callstack of exception and exact line that throws:

0:000> !ip2md 00007FFE726D8367
MethodDesc:   00007ffe71fca150
Method Name:  Microsoft.Build.Execution.ProjectInstance..ctor(Microsoft.Build.Execution.ProjectInstance, Boolean, Microsoft.Build.Execution.RequestedProjectState)
Class:        00007ffe7221bdc8
MethodTable:  00007ffe72018e50
mdToken:      00000000060015c7
Module:       00007ffe71fb1000
IsJitted:     yes
CodeAddr:     00007ffe726d79b0
Transparency: Critical
**Source file:  /_/src/Build/Instance/ProjectInstance.cs @ 848**

and the ProjectInstance with _environmentVariableProperties = null

0:000> !do 0000020eeb96b220
Name:        Microsoft.Build.Execution.ProjectInstance
MethodTable: 00007ffe72018e50
EEClass:     00007ffe7221bdc8
Size:        256(0x100) bytes
File:        C:\Program Files\Microsoft Visual Studio\18\Preview\MSBuild\Current\Bin\Microsoft.Build.dll
Fields:
              MT    Field   Offset                 Type VT     Attr            Value Name
00007ffe720565b0  400079f        8 ...Microsoft.Build]]  0 instance 0000000000000000 _actualTargets
00007ffe720564e8  40007a0       10 ...Microsoft.Build]]  0 instance 0000000000000000 _targets
00007fff11f93348  40007a1       18 ...tring, mscorlib]]  0 instance 0000000000000000 _defaultTargets
00007fff11f93348  40007a2       20 ...tring, mscorlib]]  0 instance 0000000000000000 _initialTargets
00007fff11f88eb0  40007a3       28 ...tring, mscorlib]]  0 instance 0000000000000000 _importPaths
00007fff11f88eb0  40007a4       30 ...tring, mscorlib]]  0 instance 0000000000000000 _importPathsIncludingDuplicates
00007ffe7201de08  40007a5       38 ...Microsoft.Build]]  0 instance 0000020eeb96b320 _globalProperties
00007fff106d03a0  40007a6       40 ...tring, mscorlib]]  0 instance 0000000000000000 _globalPropertiesToTreatAsLocal
00007fff11f9c638  40007a7       f4       System.Boolean  1 instance                0 _explicitToolsVersionSpecified
00007ffe7201de08  40007a8       48 ...Microsoft.Build]]  0 instance 0000020eeb96bee0 _properties
00007ffe7201de08  40007a9       50 ...Microsoft.Build]]  **0 instance 0000000000000000 _environmentVariableProperties**
00007ffe7201de08  40007aa       58 ...Microsoft.Build]]  0 instance 0000000000000000 _sdkResolvedEnvironmentVariableProperties
00007ffe72079e88  40007ab       60 ...Microsoft.Build]]  0 instance 0000020eeb96c890 _items
00007ffe7207b3a8  40007ac       68 ...Microsoft.Build]]  0 instance 0000000000000000 _itemsByEvaluatedInclude
00007fff11fa07a0  40007ad       70        System.String  0 instance 0000000000000000 _directory
00007ffe72025028  40007ae       78 ...n.ElementLocation  0 instance 0000000000000000 _projectFileLocation
00007ffe720797f8  40007af       80 ...Microsoft.Build]]  0 instance 0000000000000000 _itemDefinitions
00007ffe7202af50  40007b0       88 ...tion.HostServices  0 instance 0000000000000000 _hostServices
00007fff11f9c638  40007b1       f5       System.Boolean  1 instance                0 _usingDifferentToolsVersionFromProjectFile
00007fff11fa07a0  40007b2       90        System.String  0 instance 0000000000000000 _originalProjectToolsVersion
00007fff11f9c638  40007b3       f6       System.Boolean  1 instance                1 _isImmutable
00007ffe7205ac40  40007b4       98 ...ild]], mscorlib]]  0 instance 0000000000000000 _beforeTargets
00007ffe7205ac40  40007b5       a0 ...ild]], mscorlib]]  0 instance 0000000000000000 _afterTargets
00007ffe72021a70  40007b6       a8 ...valuation.Toolset  0 instance 0000000000000000 _toolset
00007fff11fa07a0  40007b7       b0        System.String  0 instance 0000000000000000 _subToolsetVersion
00007ffe720307d0  40007b8       b8 ...tion.TaskRegistry  0 instance 0000000000000000 _taskRegistry
00007fff11f9c638  40007b9       f7       System.Boolean  1 instance                0 _translateEntireState
00007fff11fa3368  40007ba       f0         System.Int32  1 instance               -1 _evaluationId
00007ffe7202bc58  40007bb       c0 ...estedProjectState  0 instance 0000020eeb96c438 _requestedProjectStateFilter
00007ffe720568e0  40007bc       c8 ...Microsoft.Build]]  0 instance 0000000000000000 <EvaluatedItemElements>k__BackingField
00007fff11f88f10  40007bd       d0 ...tring, mscorlib]]  0 instance 0000000000000000 <ImportPaths>k__BackingField
00007fff11f88f10  40007be       d8 ...tring, mscorlib]]  0 instance 0000000000000000 <ImportPathsIncludingDuplicates>k__BackingField
00007ffe7201f388  40007bf       e0 ...tElementCacheBase  0 instance 0000000000000000 <ProjectRootElementCache>k__BackingField
00007ffe72030320  40007c0       e8 ...ng.LoggingContext  0 instance 0000000000000000 _loggingContext

Testing

Manual