When there is only one translation variant, always select it

Summary:
Some translation parameters (like HTML strings) don't have variants.

Some languages don't have variants for certain parameters (like English for the sex of a person).

In these cases, don't require translations to provide variants.

Test Plan: Added and ran unit test.

Reviewers: chad, vrana

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D4985