Bug 833213 - [Email] Search breaks on empty subject; visible in "subject" and "all" search filters. r=squib

land https://github.com/mozilla-b2g/gaia-email-libs-and-more/pull/118
https://bugzilla.mozilla.org/show_bug.cgi?id=833213
Merge pull request #7910 from asutherland/email-search-null-subjects-master

Bug 833213 - [Email] Search breaks on empty subject; visible in "subject" and "all" search filters. r=squib