Fix -Wnullable-to-nonnull-conversion errors

Aosp/2566619 added Nonnull annotations in strcasecmp, which causes build
break in DNS resolver module and tests.

This commit:
1. Checks if a nullable variable is null before passing it to the
   strcasecmp.
2. Adds a hacky way to avoid -Wnullable-to-nonnull-conversion errors.

Bug: 278513807
Test: presubmit
Change-Id: I146f5b91114dd7c934c9e3fb12477e4563005c5f