Fix PHP warning on REST API GET /filters

Unhandled deprecation warning in .../core/string_api.php line 237:
'rawurlencode(): Passing null to parameter 1 ($string) of type string is
deprecated.

Add type cast, both in string_url() function and in offending caller
filter_encode_field_and_value(), in preparation for a future change of
parameter type to enforce string.

Fixes 0035852