Fix crash while expanding templates on older wikis

 * They still put the wikitext in *, which resulted in the input to
   the tokenizer being undefined.

 * Here we ensure it's a string.

 * Introduced in c4ae060af704d5c981e7c69df791b0c18a1d6567.

Bug: T75526
Change-Id: I1f4b5f6a16f0f2b25a89b64f6bd3ae8243865868