fix: only include "collection" field if field in object

[alukach]

closes #394

[vincentsarago]

should we add some tests as well ?

[bitner]

pgstac.sql is created by concatenating the separated .sql files, changes need to be made in the organized sql files

[bitner]

@alukach It doesn't look like the tests are passing here

[alukach]

@bitner Yeah, I think my fix actually doesn't work 🙃

Does anything stand out to you in the code changes that wouldn't work as I am expecting (ie not including collection by default in includes)?

--- tests/basic/collection_searches.sql.out	Mon Dec  1 19:25:07 2025
***************
*** 708,710 ****
  select collection_search('{"ids":["testcollection_1","testcollection_2"], "fields": {"include": ["title"]}}');
!  {"links": [], "collections": [{"id": "testcollection_1", "title": "My Test Collection.", "collection": null}, {"id": "testcollection_2", "title": "My Test Collection.", "collection": null}], "numberMatched": 2, "numberReturned": 2}
!
--- 60,61 ----
  select collection_search('{"ids":["testcollection_1","testcollection_2"], "fields": {"include": ["title"]}}');
!  {"links": [], "collections": [{"id": "testcollection_1", "title": "My Test Collection."}, {"id": "testcollection_2", "title": "My Test Collection."}], "numberMatched": 2, "numberReturned": 2}
+ echo '***TEST FOR tests/basic/collection_searches.sql FAILED***'