Early out from FlagsState::GenerateFlagsToSwitchesMapping

Don't fill name_to_switch_map when enabled_entries are empty as it won't
be used.

Change-Id: Ia5c3339646073d42108588893a09a08a76197c2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3071285


Commit-Queue: Oleg Maximenko <olegmax@yandex-team.ru>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#910220}