catch unsupported signal strength query (at+csq)
catch unsupported signal strength query (at+csq)

some modems return ERROR on at+csq, which previously exploded when we
tried to regex the output of .query (==None). now, we'll check for none,
to differentiate between "unknown" (==False) and "unsupported" (==None),
while still allowing it to be tested as boolean.

adammck/pygsm