Fix usage of Tab#installed_(on_request|as_dependency)

[MikeMcQuaid]

These can return true, false or nil so adjust the signature to note this and fix the call sites to ensure we don't accidentally pass through nil values when we shouldn't.

While we're here, make a TODO to fix this bad API up in future.

Fixes #19076

[carlocab]

Probably worth fixing that TODO as soon as the new release goes out.

[MikeMcQuaid]

Probably worth fixing that TODO as soon as the new release goes out.

@carlocab It's been present for several years at this point so would need a slightly more involved/thoughtful refactoring, unfortunately.

(that's my way of saying "I won't be able to do this as soon as the new release goes out")

[MikeMcQuaid]

@carlocab It's been present for several years at this point so would need a slightly more involved/thoughtful refactoring, unfortunately.

To elaborate a bit on this: normally nil == false with these sort of APIs. In this case, it means "we don't have any stored data so: we're not sure". That means it probably doesn't make sense to either unconditionally make nil values be either true or false across the codebase.