Add null check when assigning null to AsTypeDeclaration

There are uncommon but theoretrically valid cases when the AsTypeDeclaration of a declaration is set to null. In these cases, a missing null check when setting IsSelfAssigned caused an NRE.
Adding the null check prevents this and, as a result, makes the TypeAnnotationPass in the reference resolver safer.

Author: MDoerner

Rubberduck.Parsing/Symbols/Declaration.cs

@@ -529,7 +529,7 @@ internal set
             {
                 _asTypeDeclaration = value;
                 IsSelfAssigned = IsSelfAssigned || (DeclarationType == DeclarationType.Variable &&
-                                 AsTypeDeclaration.DeclarationType == DeclarationType.UserDefinedType);
+                                 AsTypeDeclaration?.DeclarationType == DeclarationType.UserDefinedType);
             }
         }