Turn on nullable ref types for extensions (#798)

Author: tmilnthorp

CodeGen/CodeGen.csproj

@@ -4,6 +4,7 @@
     <OutputType>Exe</OutputType>
     <TargetFramework>netcoreapp2.1</TargetFramework>
     <LangVersion>latest</LangVersion>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>

CodeGen/Generators/UnitsNetGen/NumberExtensionsGenerator.cs

@@ -24,7 +24,9 @@ public override string Generate()
             Writer.WL(
 $@"
 using System;
-                        
+
+#nullable enable
+
 namespace UnitsNet.NumberExtensions.NumberTo{_quantityName}
 {{
     /// <summary>