Fix null ref on bases due to new AnyStr behavior

Author: MikhailArkhipov

src/Analysis/Ast/Impl/Types/PythonClassType.Generics.cs

@@ -205,7 +205,7 @@ ICollection<IPythonType> newBases
                 }
 
                 // Don't add generic type parameters to bases 
-                if (!(arg.Value is IGenericTypeParameter) && arg.Value is IMember member && !member.GetPythonType().IsUnknown()) {
+                if (_bases != null && !(arg.Value is IGenericTypeParameter) && arg.Value is IMember member && !member.GetPythonType().IsUnknown()) {
                     var type = member.GetPythonType();
                     // Type may be a specific type created off generic or just a type
                     // for the copy constructor. Consider 'class A(Generic[K, V], Mapping[K, V])'

src/Analysis/Ast/Impl/Types/PythonClassType.cs

@@ -199,7 +199,7 @@ public IReadOnlyList<IPythonType> Mro {
                 if (_mro != null) {
                     return _mro;
                 }
-                if (_bases.Count == 0) {
+                if (_bases == null || _bases.Count == 0) {
                     return new IPythonType[] { this };
                 }
                 _mro = new IPythonType[] { this };