Fix required=True not accounted for for simple argument

CommandHandler.__parse_args__ checks for the None value returned Argument.match() to possibly raise
if no value got passed to a *required* argument. Unfortunately, SimpleArgument.match() never
returns None but would instead return the empty string it receives (which is being stripped by
the caller, __parse_args__()). As such, `@argument(..., required=True)` does not work and silently
passes an empty string to the command handler.

We fix this returning None early in SimpleArgument.match() when it receives an empty string value.

maubot/handlers/command.py

@@ -403,6 +403,8 @@ class CustomArgument(Argument):
 
 class SimpleArgument(Argument):
     def match(self, val: str, **kwargs) -> Tuple[str, Any]:
+        if not val:
+            return "", None
         if self.pass_raw:
             return "", val
         res = re.split(r"\s", val, 1)[0]