Schema: Fix bug with enum

[mhuang74]

Should only add null to Enum list when it's not empty. Enum list would be empty when column has cardinality larger than threshold.

Integration test now covers this case.

$ head -50000 NYC_311_SR_2010-2020-sample-1M.csv > NYC-short.csv
$ qsvlite index NYC-short.csv
$ time qsvlite schema NYC-short.csv --value-constraints --enum-threshold=25
Schema written to NYC-short.csv.schema.json

real	1m10.350s
user	2m59.430s
sys	0m3.241s
$ time qsvlite validate NYC-short.csv NYC-short.csv.schema.json 
[00:03:45] [==================== 100% validated 49,999 records.] (222/sec)
0 out of 49,999 records invalid.

real	3m45.273s
user	3m44.656s
sys	0m0.360s