Twenty Fifteen: Check if we're in the customizer before we avoid outputting HTML for an empty tagline. Avoids an issue with a not-updating customizer preview of the tagline if you're going in with an empty tagline.

Props philiparthurmoore, celloexpressions, fixes #30358.



git-svn-id: https://develop.svn.wordpress.org/trunk@30391 602fd350-edb4-49c9-b593-d223f7449a82