Avoid extra isinstance calls in _visit_generic (#2502)

Author: correctmost

astroid/transforms.py

@@ -104,11 +104,13 @@ def _visit_generic(
     def _visit_generic(self, node: nodes.NodeNG) -> SuccessfulInferenceResult: ...
 
     def _visit_generic(self, node: _Vistables) -> _VisitReturns:
+        if not node:
+            return node
         if isinstance(node, list):
             return [self._visit_generic(child) for child in node]
         if isinstance(node, tuple):
             return tuple(self._visit_generic(child) for child in node)
-        if not node or isinstance(node, str):
+        if isinstance(node, str):
             return node
 
         try: