mw.language.convertPlural: Check if matching form exists

Same reason as for d510d0c0c7dd: return the empty string instead
of undefined when no matching form exists, as intended.

Follows-up 710d3bba3840.

Change-Id: I52df4c88d822cdf5f318651a08f1f120ea924e9c
Merge "mw.language.convertPlural: Check if matching form exists"