wfBaseConvert(): Avoid PHP warning when converting zero

The warning occurs when gmp_init() tries to convert the empty string
that came from ltrim(). This is causing tests to fail under HHVM 3.6.1.

Follows-up 5957856c46c3.

Bug: T98882
Change-Id: Ica86c91d7897db979e09d7cfc82fb3a20b95d4ce