LineageSettingsService: prevent setting net.hostname to null

If the user did not set a custom hostname, the settings service will
set net.hostname to null.

Only set net.hostname if the user actually set a custom hostname.

Change-Id: Ie9cc84e5e8c9468a7e698896e2b21c14a37ebffc
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>