NEXUS-5734: Dump empty prefix files

Unsure is this correct, or in general, is it correct
to have empty prefix file published (repo is empty?).
Typo fix
Added UT
NEXUS-5734: Ahem, wrongly placed condition

And UT actually tests it IS thrown, so passed
and not caught, but all the other UTs did.
NEXUS-5734: Reworked imple

As initial imple was pretty much to strict.
So now the constraint is applied only to files
got from remote, as it was initially intended
Merge pull request #30 from sonatype/nexus-5734-dump-empty-prefix-files

NEXUS-5734: Prevent load and use of empty prefix.txt got from remote