Support empty external compactions

I thought of a fairly simple way to implement empty compactions w/o riddling the code w/ special cases for the empty set. Using a set with a special URI for this case instead of implementing special handling for the empty set made the changes quite small. I made the changes in the following branch and the test pass now.

https://github.com/keith-turner/accumulo/tree/accumulo-1647

These changes build on the changes in #1649.

_Originally posted by @keith-turner in https://github.com/apache/accumulo/issues/1647#issuecomment-653278435_

Start by reviewing the linked accumulo-1647 branch and the changes in #1649 to understand the proposed empty-compaction approach; the issue names no files or tests. Confirm the relevant compaction behavior and test coverage, with support for empty external compactions as the completion criterion.

Written by the indexing model from the issue text.