Handle 204 no content for SingleImageLayer

[arnaudboudier-sogelink]

When receiving a 204 HTTP status code in tiled requests, Mapfish handles it by inserting a transparent tile. This is not the case for SingleImageLayer (such as WMS), where it results in an error.
I have implemented the same behavior as for tiled requests in this PR.

[arnaudboudier-sogelink]

@sbrunner I have run the UT using make tests on my system without problem.
The problem in the CI is linked to this PR ? #3885 ?

[sebr72]

@arnaudboudier-sogelink Indeed there was a pb with the tests. So Gradle 9 commit was "remerged" in master.

[arnaudboudier-sogelink]

I have rebased my branch @sebr72 can you check that everything is OK ?

[sebr72]

@arnaudboudier-sogelink I had a look at you PR, and I would approve it. But we had started the migration of Spring . So we have to freeze accepting new PRs, to keep our costs down.
Sorry for the delay. But it will be much easier to merge your PR on top of Spring rather than replay your changes in ours.

[arnaudboudier-sogelink]

Ok no problem, do you know if it will be possible (once my commit will be merged after your spring migration), to backport it also to 3.33 version ?

[sebr72]

@arnaudboudier-sogelink I am not sure how we will proceed to get this PR back-ported to 3.33, but I am sure we will find a solution even if it requires extra work (since you created it before Spring 7 was in master)