Return empty item collection instead of error when searching

[zstatmanweil]

This PR addresses issue #172.

• A POST to /search that returns no items now returns an empty list of features rather than a 404 and NotFoundError;
• I kept the logic that a GET /collections/{collectionId}/items with an invalid collection ID still returns a 404 rather than a 200 with an empty features list
• I added logic such that a GET to /collections/{collectionId}/items/{itemId} with an invalid collection ID returns a 404

Testing:

• Added a few unit tests and all unit tests are passing
• Ran locally and tested behavior as expected

cc: @moradology, @lossyrob

[moradology]

I found an incredibly minor issue and one bit that I had some questions about, but this PR looks good to me. Nice tests in particular.

[moradology]

I'm curious what convinced you to safeguard against non-existent collections here. Maybe you saw some performance hits in that case which can be avoided?

[zstatmanweil]

This addition is to maintain the existing logic that a request to this endpoint would result in an error if a collection doesn't exist. Previously it was the item_collection = await self._search_base(req, **kwargs) that would result in a 404 since no items would be found, but since that now returns an empty collection and 200 if no items are found, I wanted to make sure an error was raised.

I was most unsure about this change, especially since The Planetary Computer STAC actually doesn't raise a 404 in this instance when a collection doesn't exist. I think logically it should though. What do you think?

[lossyrob]

Good catch - the PC should def return a 404 instead of an empty collection in this case, so this change LGTM

[moradology]

Same question as above regarding the benefits of this safeguard

[zstatmanweil]

Same idea as above - checking if a collection exists because if not, I think a 404 should be returned