DIH: last_index_time not updated on if 0 docs updated

After getting our interval for calling delta index shorter and shorter, I
have found out that last_index_time in dataimport.properties is not
updated every time the indexing runs, it is skipped if no docs where added.

This happens at least in the following scenario:

• running delta as full index
  ( /dataimport?command=full-import&clean=false&commit=true )
• Solrcloud setup, so dataimport.properties is in zookeeper
• Solr 5.5.0

I understand skipping the commit on the index if no docs were updated is a
nice optimization, but I believe the last_index_time info should be updated
in all cases, so it reflects reality. We, for instance, are looking at this
piece of information in order to do other stuff.