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
API: Fix parameter validation in setnotificationtimestamp

This was broken in I7a3d7b6e, when the ApiPageSet parameters stopped
being returned by getAllowedParams() and so by extractRequestParams().
Although it would be broken differently if they had been.

Change-Id: I4b6ec21fd7b7c932856546df1ccad574d996db1f
Merge changes I4b6ec21f,I97556dbc into REL1_21

* changes:
  API: Fix parameter validation in setnotificationtimestamp
  CoreParserFunctions::anchorencode should return a string