Allow for nullable ref param in SearchBarCommands types

This is not breaking as it allows for wider gamut of types to be
accepted.

Ideally we should allow for `React.Ref` there, as suggested here:
#2724 (comment),
but it'd require some more internal changes.

Author: kkafar

src/types.tsx

@@ -832,7 +832,7 @@ export interface SearchBarProps {
    * * `cancelSearch` - cancel search in search bar.
    * * `toggleCancelButton` - depending on passed boolean value, hides or shows cancel button (iOS only)
    */
-  ref?: React.RefObject<SearchBarCommands>;
+  ref?: React.RefObject<SearchBarCommands | null>;
 
   /**
    * The auto-capitalization behavior.