Fix crash in network service webui code when the source gives an empty string.

This is handled correctly by the old webui case.

BUG: 778842
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I39013667b9038e9687b418905e56e93868017b24
Reviewed-on: https://chromium-review.googlesource.com/781419


Reviewed-by: Yuzhu Shen <yzshen@chromium.org>
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518275}