Make ApplicationBuilder play better with nullable

[pranavkm]

Capturing feedback from #23091 (comment)

1. Adding a parameterless constructor would simplify usage in tests.
2. ApplicationServices_Get should return a static no-op instance rather than null.
3. ServerFeatures should also return a read-only no-op instance.

[pranavkm]

Closing this. We don't need an issue to write better code.