Fix crash: Skip SetDNREnabledStaticRulesets if matcher is nullptr

In the OnNewStaticRulesetsLoaded(), load_data.rulesets can be empty
when the updateEnabledStaticRulesets() is called with empty
ids_to_enable (only having ids_to_disable).

If load_data.rulesets is empty and the extension didn't have any
existing rulesets, we don't need to update the DNREnabledStaticRulesets
since it will not be changed. (It was empty list before and it is still
empty)

In this case, we can skip SetDNREnabledStaticRulesets if matcher is nullptr to avoid crash.

Bug: 1354385
Change-Id: Iaf25be36135413ec83399e2193c2ce2ed96f4a94
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3843278
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Commit-Queue: Byungwoo Lee <blee@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1038526}