Translate string iff non-empty

This resolves an issue where a checklist's long_description being empty
caused gettext to dump PO file headers.

I had originally made this change to the Stanford-Online/edx-platform
repository a few weeks ago [0] to address a failing test on our fork.
I didn't take the time to diagnose the root issue; I just fixed it for
the single case.

After stumbling across the issue elsewhere, @kluo <kevkluo@gmail.com>
followed through and tracked down the cause of the behavior. It turns
out that `gettext`, by default, translates the empty string into PO file
headers; this is apparently a feature, not a bug [1].

This behavior seems counterintuitive; for our use, it seems that
translating an empty string should simply return an empty string. We
already program defensively around this in platform code [2], but it'd be
convenient to resolve this with a centralized solution, instead of a
patchwork of fixes.

[0] 4b88f54aa658db952e899222ff5ede4a1c1beb18
[1] http://stackoverflow.com/questions/18459387/translation-of-empty-string-shows-po-header
[2] https://github.com/edx/edx-platform/blob/master/cms/djangoapps/contentstore/views/checklist.py#L140
Basic notifications handling.

LMS-11163
bok-choy: check mysql the old way.
Merge pull request #4658 from edx/benp/fix-bok-choy-check

bok-choy: check mysql the old way.
Revert "allow rsyslog to be none"
Revert "bok-choy: check mysql the old way."
Getting data package files from AWS
Merge pull request #4659 from edx/revert-4652-jarv/only-add-handler-if-rsyslog

Revert "allow rsyslog to be none"
Merge pull request #4572 from edx/ahodges/documentation/DOC764

Ahodges/documentation/doc764
numbering error
Merge pull request #4661 from edx/ahodges/documentation/numbering

numbering error
Merge pull request #4641 from edx/reruns/server-side-handlers

Basic notifications handling.
Merge pull request #4653 from Stanford-Online/stv/i18n/empty

Translate string iff non-empty
Merge pull request #4660 from edx/revert-4658-benp/fix-bok-choy-check

Revert "bok-choy: check mysql the old way."
BLD 1020: Easy access to Group Configurations page from Content Experiment component.
Addressed comments.
Added Jasmine tests and extra check on erroneous id in url hash.
Addressed further comment.
REVERT THIS COMMIT BEFORE MERGE.
Fix test and address last comments.