stbt.py: Fix race condition: TypeError on NoVideo

During a NoVideo source pipeline reset `Display.source_pipeline` is
`None`.  If simultaneously `get_sample` times out stbt will try to dump
the source pipeline graph.  This causes `TypeError` to be raised:

      File ".../stbt/stbt.py", line 1493, in get_sample
        self.source_pipeline, Gst.DebugGraphDetails.ALL, "NoVideo")
    TypeError: Argument 0 does not allow None as a value

Thanks to Lewis Haley for reporting this bug and correctly diagnosing the
cause.

Fixes drothlis/#197