(PDB-1076) Make environment garbage collection ignore null values

If a factset, report or catalog is migrated from an earlier puppetdb
version or submitted with a earlier puppetdb-terminus version it has
NULL as environment_id. If any of them have a record with that PuppetDB
failed to garbage collect any environments at all as the subselect
returned a NULL value.

Switching from UNION ALL to just UNION speeds up the query dramatically
as the return value from the subselect is much smaller (doesn't contain
any duplicates).
Merge pull request #1184 from dalen/pdb-1076-environment-gc-query

(PDB-1076) Make environment garbage collection ignore null values