Fix IpClientIntegrationTests on Q

On Q if no hostname is configured, the legacy behavior of reading a
hostname from system properties is preserved. In most cases this causes
the hostname to be an empty string, not null.
Fix the test to expect the right hostname on Q devices.

Test: atest NetworkStackIntegrationTests on a Q device
Change-Id: I7c914c9cec8c3ca564e8be7e980da4ef054a3753