* Allow strings to be empty in metadata

not sure why it originally required a nil or a non-blank string. But
when you want to override a description and clear it out, the front-end
text boxes will send over an empty string "".

This value is useful on its own though. Particularly in mbql queries or
in native models where you've identified an underlying field, it will
merge the metadata of the underlying field with the metadata of the
model. So if we keep a nil there the underlying can show up. Especially
for mbql queries, where you have the underlying description to begin
with, you clear it out and then it appears to remain. Because of this
merging. Using an affirmative "" empty string value prevents the merge
and we all are happy

* Lets also fix #11097 while we're near

not identical, but same problem enforced at the route rather than some
schema though

* Unneeded thread macro