getInlineStyleOverride(): ?DraftInlineStyle {
    return this.getImmutable().get('inlineStyleOverride');
  }
While editing, the user may apply inline style commands with a collapsed
cursor, intending to type text that adopts the specified style. In this
case, we track the specified style as an "override" that takes precedence
over the inline style of the text adjacent to the cursor.

If null, there is no override in place.