Fix Null Exception crash

ProjectId for HostArgId can be null for workspace diagnostic exception. make sure we guard from it.

src/Features/Core/Diagnostics/AbstractHostDiagnosticUpdateSource.cs

@@ -155,12 +155,12 @@ internal ImmutableHashSet<DiagnosticData> TestOnly_GetReportedDiagnostics(Diagno
         private class HostArgsId : AnalyzerUpdateArgsId
         {
             private readonly AbstractHostDiagnosticUpdateSource _source;
-            private readonly ProjectId _projectId;
+            private readonly ProjectId _projectIdOpt;
 
-            public HostArgsId(AbstractHostDiagnosticUpdateSource source, DiagnosticAnalyzer analyzer, ProjectId id) : base(analyzer)
+            public HostArgsId(AbstractHostDiagnosticUpdateSource source, DiagnosticAnalyzer analyzer, ProjectId projectIdOpt) : base(analyzer)
             {
                 this._source = source;
-                this._projectId = id;
+                this._projectIdOpt = projectIdOpt;
             }
 
             public override bool Equals(object obj)
@@ -171,12 +171,12 @@ public override bool Equals(object obj)
                     return false;
                 }
 
-                return _source == other._source && _projectId == other._projectId && base.Equals(obj);
+                return _source == other._source && _projectIdOpt == other._projectIdOpt && base.Equals(obj);
             }
 
             public override int GetHashCode()
             {
-                return Hash.Combine(_source.GetHashCode(), Hash.Combine(_projectId.GetHashCode(), base.GetHashCode()));
+                return Hash.Combine(_source.GetHashCode(), Hash.Combine(_projectIdOpt == null ? 1 : _projectIdOpt.GetHashCode(), base.GetHashCode()));
             }
         }
     }