avoid relatedTemplate ever returning ""

avoid relatedTemplate ever returning ""

add: Fix crash adding filenames that are exactly 21 bytes long and begin
with a utf-8 character.

Also longer filenames that start with "....." would cause the same crash.

I also audited for other calls to truncateFilePath that could truncate it
to "". Most use pathmax so are not a problem. Backend.Utilities.genKeyName
could possibly truncate it like that, but appends the md5 so would not be a
problem either.

Sponsored-by: Kevin Mueller