Fix new MyPy issues

Left operand of "or" is always false  [redundant-expr]

zarr/_storage/store.py

@@ -196,7 +196,7 @@ def _valid_key(self, key: str) -> bool:
         0-9, or in the set /.-_ it will return True if that's the case, False
         otherwise.
         """
-        if not isinstance(key, str) or not key.isascii():
+        if not isinstance(key, str) or not key.isascii():  # type: ignore[redundant-expr]
             return False
         if set(key) - self._valid_key_characters:
             return False

zarr/storage.py

@@ -2501,6 +2501,8 @@ def _accommodate_value(self, value_size):
             self._current_size -= buffer_size(v)
 
     def _cache_value(self, key: Path, value):
+        if self._max_size is None:
+            return
         # cache a value
         value_size = buffer_size(value)
         # check size of the value against max size, as if the value itself exceeds max