Allow `NULL` as an exception value when return value is a pointer to a fused type

Cython/Compiler/ExprNodes.py

@@ -850,6 +850,9 @@ class ExprNode(Node):
 
             if src_type.is_fused:
                 error(self.pos, "Type is not specialized")
+            elif src_type.is_null_ptr and dst_type.is_ptr:
+                # NULL can be implicitly cast to any pointer type
+                return self
             else:
                 error(self.pos, "Cannot coerce to a type that is not specialized")