fix tests

Author: KurosawaAngel

src/aiogram_dialog/context/media_storage.py

@@ -26,7 +26,7 @@ def _get_file_mtime(self, path: Optional[str]) -> Optional[float]:
         if not path:
             return None
         p = Path(path)
-        if p.exists():
+        if not p.exists():
             return None
         return Path(path).stat().st_mtime