Revert a return value.

Not sure why mypy tuple is okay here but Optional[tuple] is not.
some return paths return None

Author: rocky

mathics/builtin/drawing/plot.py

@@ -387,7 +387,7 @@ def get_plotrange(self, plotrange, start, stop):
 
     def process_function_and_options(
         self, functions, x, start, stop, evaluation: Evaluation, options: dict
-    ) -> Optional[tuple]:
+    ) -> tuple:
         """Process the arguments of a plot expression."""
         if isinstance(functions, Symbol) and functions.name is not x.get_name():
             rules = evaluation.definitions.get_ownvalues(functions.name)