Fix assertion: UnknownType for unresolved reference can still be nullable; only the SpecialType.Unknown cannot.

ICSharpCode.Decompiler/TypeSystem/Implementation/NullabilityAnnotatedType.cs

@@ -20,6 +20,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			// the NullabilityAnnotatedType wrapper only in some limited places.
 			Debug.Assert(type is ITypeDefinition { IsReferenceType: not false }
 				|| type.Kind == TypeKind.Dynamic
+				|| type.Kind == TypeKind.Unknown
 				|| (type is ITypeParameter && this is ITypeParameter));
 			this.nullability = nullability;
 		}

ICSharpCode.Decompiler/TypeSystem/Implementation/UnknownType.cs

@@ -107,7 +107,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 
 		public override IType ChangeNullability(Nullability nullability)
 		{
-			if (nullability == Nullability.Oblivious)
+			if (nullability == Nullability.Oblivious || isReferenceType == false)
 				return this;
 			else
 				return new NullabilityAnnotatedType(this, nullability);