Bug 991949 - Cell broadcast ETWS messages show undefined

ETWS messages without a message body use a canned body depending on the
warning type.

Change-Id: I25491fd85894d334bd6a9ab63105379de495ef2a
Merge pull request #19899 from mschwart/bug991949

Bug 991949 - Cell broadcast ETWS messages show undefined, r=timdream