Allow pd.StringDtype's optional arg (#535)

Fixed pd.StringDtype's optional arg with test case

Author: swimninja247

pandas-stubs/core/arrays/string_.pyi

@@ -7,7 +7,7 @@ from pandas._libs.missing import NAType
 from pandas.core.dtypes.base import ExtensionDtype
 
 class StringDtype(ExtensionDtype):
-    def __init__(self, storage: Literal["python", "pyarrow"] | None) -> None: ...
+    def __init__(self, storage: Literal["python", "pyarrow"] | None = None) -> None: ...
     @property
     def na_value(self) -> NAType: ...
 

tests/test_dtypes.py

@@ -126,6 +126,7 @@ def test_sparse_dtype() -> None:
 
 def test_string_dtype() -> None:
     s_dt = pd.StringDtype("pyarrow")
+    check(assert_type(pd.StringDtype(), pd.StringDtype), pd.StringDtype)
     check(assert_type(pd.StringDtype("pyarrow"), pd.StringDtype), pd.StringDtype)
     check(assert_type(pd.StringDtype("python"), pd.StringDtype), pd.StringDtype)
     check(assert_type(s_dt.na_value, NAType), NAType)