ENH: add dtype validation for value in StringArray.fillna

Author: arw2019

pandas/core/arrays/string_.py

@@ -283,7 +283,8 @@ def __setitem__(self, key, value):
         super().__setitem__(key, value)
 
     def fillna(self, value=None, method=None, limit=None):
-        # TODO: validate dtype
+        if not isinstance(value, str):
+            raise TypeError(f"{value} is not a valid fill value; must be a string")
         return super().fillna(value, method, limit)
 
     def astype(self, dtype, copy=True):