wrong logic for flag check

src/pint/toa.py

@@ -955,7 +955,7 @@ def check_allowed_key(k: Any) -> None:
     def check_allowed_value(k: str, v: Any) -> None:
         if not isinstance(v, str):
             raise ValueError(f"value {v} for key {k} must be a string")
-        if not v and len(v.split()) != 1:
+        if len(v.split()) != 1:
             raise ValueError(f"value {repr(v)} for key {k} cannot contain whitespace")
 
     def __setitem__(self, key: str, val: str):