JERSEY-2486: Clarify Javadoc of PatternWithGroups.getGroupIndexes()

 - updated javadoc
 - UriTemplateParser.getGroupIndexes now returns non-empty array in
   case no nested capturing groups are used in a template
 - test

Change-Id: Ie09b1da069b3dfcb2f7dc1a66b844fb8c1519019
Signed-off-by: Michal Gajdos <michal.gajdos@oracle.com>