py,st3m: BaseView: Guard the usage of bundled InputController instance

Allows users to disable the provided InputController instance with
`self.input = None`.

python_payload/st3m/ui/view.py

@@ -66,10 +66,12 @@ class BaseView(View):
         self.vm: Optional["ViewManager"] = None
 
     def on_enter(self, vm: Optional["ViewManager"]) -> None:
+        if self.input:
             self.input._ignore_pressed()
         self.vm = vm
 
     def think(self, ins: InputState, delta_ms: int) -> None:
+        if self.input:
             self.input.think(ins, delta_ms)
 
     def is_active(self) -> bool: