Saving a translated message template with no text version fails
The text version should not be necessary - and in fact we have general agreement to always send both
However, when we go to save it fatals because it attempts to save the text version
This is slightly complicated because the fallback for 'no text version exists' should be 'use the relevant html' not 'fall back on default language;' - so we should probably save an empty string or null or something