Fix nulability

Author: rokonec

src/Build/Logging/BinaryLogger/BuildEventArgsReader.cs

@@ -947,7 +947,7 @@ private void ReadDiagnosticFields(BuildEventArgsFields fields)
         private ExtendedDataFields? ReadExtendedDataFields()
         {
             string extendedType = ReadOptionalString()!;
-            IDictionary<string, string>? extendedMetadata = ReadStringDictionary();
+            IDictionary<string, string?>? extendedMetadata = ReadStringDictionary()!;
             string? extendedData = ReadOptionalString();
 
             return new ExtendedDataFields(extendedType, extendedMetadata, extendedData);

src/Build/Logging/BinaryLogger/ExtendedDataFields.cs

@@ -7,7 +7,7 @@ namespace Microsoft.Build.Logging;
 
 internal class ExtendedDataFields
 {
-    public ExtendedDataFields(string extendedType, IDictionary<string, string>? extendedMetadata, string? extendedData)
+    public ExtendedDataFields(string extendedType, IDictionary<string, string?>? extendedMetadata, string? extendedData)
     {
         ExtendedType = extendedType;
         ExtendedMetadata = extendedMetadata;