try fix typing

Author: jorisvandenbossche

pandas/core/arrays/string_.py

@@ -122,7 +122,9 @@ def na_value(self) -> libmissing.NAType | float:  # type: ignore[override]
 
     _metadata = ("storage",)
 
-    def __init__(self, storage=None, na_value=libmissing.NA) -> None:
+    def __init__(
+        self, storage=None, na_value: libmissing.NAType | float = libmissing.NA
+    ) -> None:
         if not (
             na_value is libmissing.NA
             or (isinstance(na_value, float) and np.isnan(na_value))