try fix typing

Author: jorisvandenbossche

pandas/core/arrays/string_.py

@@ -297,12 +297,14 @@ def _get_common_dtype(self, dtypes: list[DtypeObj]) -> DtypeObj | None:
             else:
                 return None
 
+        storage: str
         if len(storages) == 2:
             # if both python and pyarrow storage -> priority to pyarrow
             storage = "pyarrow"
         else:
             storage = next(iter(storages))
 
+        na_value: libmissing.NAType | float
         if len(na_values) == 2:
             # if both NaN and NA -> priority to NA
             na_value = libmissing.NA