Docvalues sorting treats empty values as the default [LUCENE-7548]

I found this working on Solr, Mike suggested I raise it as a Lucene issue.

This was for int fields, I'll assume the same holds for other types.

The sort order is as though the null field was explicitly set to zero, the whole idea of sorting missing first is lost.

doc1
int_dv = 0
int_sort = 1

doc2
int_sort=2

doc3
int_dv = 0
int_sort=3

sort by int_dv asc,int_sort asc gives
doc1
doc2
doc3

when it should give
doc2
doc1
doc3

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Migrated from [LUCENE-7548](https://issues.apache.org/jira/browse/LUCENE-7548) by Erick Erickson (@ErickErickson)
Linked issues:
- [SOLR-9166](https://issues.apache.org/jira/browse/SOLR-9166)

No source file or test is identified in the issue. Start by reproducing the documented int_dv and int_sort example with ascending sorting, then trace the docvalues sorting path responsible for missing values. Done means missing values sort first, producing doc2, doc1, doc3, with equivalent behavior checked for the other types.

Written by the indexing model from the issue text.