[RSG] Cleanup SuggestionGenerators

This CL:
1. Renames the SuggestionGenerator interface aligning it with broader
autofill naming standards.
2. Adds a regression test, ensuring that AutofillAiSuggestionGenerator
does not return any suggestion data if entity does not have any value
to fill for a given field.

Bug: 409962888
Change-Id: I355e2b4ef2a4f53533e1aaf6d37517be9d307e7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6909856
Reviewed-by: Jihad Hanna <jihadghanna@google.com>
Commit-Queue: Karol Sygiet <sygiet@google.com>
Cr-Commit-Position: refs/heads/main@{#1510814}