Bug 867948 - Ensure findByPhoneNumber callback checks their values

This is legitimate (and it seems meaningfull) for findByPhoneNumber to
return either null or empty array, depending on the case. However, we
must ensure to properly check for those returns values, otherwise we
might blow up in the air.
Merge pull request #10019 from lissyx/bug867948

Bug 867948 - Ensure findByPhoneNumber callback checks their values
r=julien