Fix {{#property:}} calls for missing data on wikis with LanguageConverter

It was producing a list of all variant codes, because output strings are
empty for each variant, and the converter doesn't handle empty strings as
conversion entries correctly, having raw conversion code (except for the
surrounding -{ }-) output (like "sr:;sr-ec:;sr-el:;") as is.

Change-Id: Iada5fbd8736753622af734e3494d0b3fa0109f68
Merge "Fix {{#property:}} calls for missing data on wikis with LanguageConverter"