Fix exceptions when running code fix tests in StyleCopTester

Closes #1991
Closes #1992
Closes #1993
Closes #1994
Closes #1995
Closes #1996
Closes #1997

Author: sharwell

StyleCop.Analyzers/StyleCopTester/TesterDiagnosticProvider.cs

@@ -24,7 +24,19 @@ public TesterDiagnosticProvider(ImmutableDictionary<ProjectId, ImmutableDictiona
 
         public override Task<IEnumerable<Diagnostic>> GetAllDiagnosticsAsync(Project project, CancellationToken cancellationToken)
         {
-            return Task.FromResult(this.projectDiagnostics.Values.SelectMany(i => i).Concat(this.documentDiagnostics.Values.SelectMany(i => i.Values).SelectMany(i => i)));
+            ImmutableArray<Diagnostic> filteredProjectDiagnostics;
+            if (!this.projectDiagnostics.TryGetValue(project.Id, out filteredProjectDiagnostics))
+            {
+                filteredProjectDiagnostics = ImmutableArray<Diagnostic>.Empty;
+            }
+
+            ImmutableDictionary<string, ImmutableArray<Diagnostic>> filteredDocumentDiagnostics;
+            if (!this.documentDiagnostics.TryGetValue(project.Id, out filteredDocumentDiagnostics))
+            {
+                filteredDocumentDiagnostics = ImmutableDictionary<string, ImmutableArray<Diagnostic>>.Empty;
+            }
+
+            return Task.FromResult(filteredProjectDiagnostics.Concat(filteredDocumentDiagnostics.Values.SelectMany(i => i)));
         }
 
         public override Task<IEnumerable<Diagnostic>> GetDocumentDiagnosticsAsync(Document document, CancellationToken cancellationToken)