CoreParserFunctions::anchorencode should return a string

CoreParserFunctions::anchorencode incorrectly returns false rather than
the empty string when passed an empty string.

A simple cast fixes it; this likely wasn't noticed before since PHP was
automatically doing the cast anyway when the return value was merged
into wikitext.

Bug: 46608
Change-Id: I97556dbc4dcc1f102f6fed499d43dada388cdc5d
Merge "CoreParserFunctions::anchorencode should return a string"