NEXUS-5832: Extended UT with "empty XML file" case

No code change aside in UT. Validation performs well,
(empty XML file passes with "laxed" validation, fails
with "strict").

Default is lax=true.
Fix InvalidConfigurationExceptionMapper to return the converted list of exceptions

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Merge remote-tracking branch 'origin/master' into nexus-5790-new-content-impl