ethernet_interface: Only write Gateway= lines when present

This doesn't affect the total system behavior but it cleans up error
output from systemd-networkd which doesn't like empty Gateway= lines
when we don't have a set gateway.

Tested:
    Set the system to a static configuration with no gateway configured
    and saw that the line was no longer written. Configured a gateway
    and the line is written as expected.

Change-Id: I1ff9c4f564fab54d0c54e3f87fcd50b623f0fbda
Signed-off-by: William A. Kennington III <wak@google.com>
1 file changed