roswtf: do not try to run online checks if there are no roslaunch uris

[flixr]

If there are no roslaunch uris, roswtf aborts checks with a traceback:

Traceback (most recent call last):
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roswtf/__init__.py", line 220, in _roswtf_main
    roswtf.roslaunchwtf.wtf_check_online(ctx)
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roswtf/roslaunchwtf.py", line 297, in wtf_check_online
    warning_rule(r, r[0](ctx), ctx)
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roswtf/roslaunchwtf.py", line 202, in roslaunch_respawn_check
    for uri in ctx.roslaunch_uris:
TypeError: 'NoneType' object is not iterable
'NoneType' object is not iterable

So do not try to run online checks if there are no roslaunch_uris to fix this.

[flixr]

The failed one looks like a flaky test.
I see basically zero risk in this...
Can we please get this merged?

[mikepurvis]

Yup, thanks for the ping— this looks reasonable, thanks.