collection free-text search fails when optional fields are null

[hrodmn]

The logic that handles free-text queries was not working as expected when either title or keywords was null:

pgstac/src/pgstac/sql/004_search.sql

Lines 221 to 230 in 6da165b

	where_segments := where_segments || format(
	$quote$
	(
	to_tsvector('english', content->'properties'->>'description') ||
	to_tsvector('english', content->'properties'->>'title') ||
	to_tsvector('english', content->'properties'->'keywords')
	) @@ %L
	$quote$,
	ft_query
	);

The test cases that I added all had title, description, keywords filled out so the null cases were never tested!

pgstac/src/pgstac/tests/basic/free_text.sql

Lines 21 to 52 in 6da165b

	(
	'Stranger Things',
	'Some teenagers drop out of school to fight monsters',
	'monster, scary, dark, 80s',
	-180, -90, 180, 90,
	'2016-01-01T00:00:00Z',
	'2025-12-31T23:59:59Z'
	),
	(
	'The Bear',
	'Another story about why you should not start a restaurant',
	'restaurant, funny, sad, great',
	-180, -90, 180, 90,
	'2022-01-01T00:00:00Z',
	'2025-12-31T23:59:59Z'
	),
	(
	'Godzilla',
	'A large lizard takes its revenge',
	'scary, lizard, monster',
	-180, -90, 180, 90,
	'1954-01-01T00:00:00Z',
	null
	),
	(
	'Chefs Table',
	'Another great story that make you wonder if you should go to a restaurant',
	'restaurant, food, michelin',
	-180, -90, 180, 90,
	'2019-01-01T00:00:00Z',
	'2025-12-31T23:59:59Z'
	);