Fix BottomTab options types

Though icon is required on Android when declaring tabs, it's optional when calling mergeOptions to update tabs.

lib/src/interfaces/Options.ts

@@ -570,8 +570,9 @@ export interface OptionsBottomTab {
   testID?: string;
   /**
    * Set the tab icon
+   * Note: On Android `icon` is required
    */
-  icon: ImageRequireSource;
+  icon?: ImageRequireSource;
   /**
    * Set the icon tint
    */