#ir_background should return nil if URL is nil

[johnkchow]

Sometimes URLs are persisted as nil (i.e. users w/ blank avatars) and when passed into #ir_background, it'll generate a background-image: url() CSS, which causes the page to essentially reload the document again. If we simply return nil here, we avoid the accidental page reloading.

[jimmynicol]

Good catch, fixed and pushed. All ruby methods now return a nil when no valid source is provided.