fix: next_patch endpoint returns false if the latest patch does not have an artifact for that platform

[eseidel]

shorebird release android
shorebird release ios
shorebird patch android
shorebird patch ios

Your android clients will now refuse to update to patch #1 because patch #2 exists that doesn't have an android artifact.

The fact that these two patches are separate is also an issue, tracked by: #752

But both of these need fixing I believe?

[eseidel]

I suspect this could also manifest if you started a patch update but then aborted at a bad time? You could end up with a new patch N created that didn't yet have an android artifact uploaded and your android clients would then refuse to update to N-1?

Presumably this needs to be a bit smarter?
https://github.com/shorebirdtech/_shorebird/blob/main/packages/code_push_server/routes/api/v1/patches/check.dart#L106