Fix crash in the visitor implementation (#10)

It was assumed to always be inside a class, which isn't
true in a lot of cases

src/flake8_pydantic/visitor.py

@@ -28,7 +28,9 @@ def leave_class(self) -> None:
         self.class_stack.pop()
 
     @property
-    def current_class(self) -> ClassType:
+    def current_class(self) -> ClassType | None:
+        if not self.class_stack:
+            return None
         return self.class_stack[-1]
 
     def _check_pyd_001(self, node: ast.AnnAssign) -> None: