Prevent default for input events that don't have any characters

Seeing cases where some input events with `inputType=deleteContentBackward` are slipping through. These are probably being created via some mechanism other than a keydown directly on the script (e.g., through some accessibility API, or touch control, or some event that's being targeted elsewhere but propagating to the script accidentally).

We _never_ want draft.js to do anything in response to an `onInput` in our user case. We were relying on our passed-in `handleBeforeInput` to always return `handled`; but we're only hitting that logic when the input event contains characters. This will hopefully fix that.

src/component/handlers/edit/editOnBeforeInput.ts

@@ -116,6 +116,7 @@ export default function editOnBeforeInput(
   // In some cases (ex: IE ideographic space insertion) no character data
   // is provided. There's nothing to do when this happens.
   if (!chars) {
+    e.preventDefault();
     return;
   }