WifiPlugin.get_alert chained its three comparisons, so the first undefined level
was compared against None, raised TypeError, and the blanket handler dropped the
result to DEFAULT. Because 'critical' is tested first, leaving it out silenced
the other two: a config defining only wifi_careful produced no alert at all, and
even a strong signal came back undecorated instead of OK.
GlancesPluginModel.get_alert already guards each level separately, and the
sensors plugin has tests pinning that partial thresholds work. This brings wifi
in line. `is not None` rather than truthiness because these limits are negative
dBm values, where `if threshold` would discard a 0.
The TypeError handler is kept for a non-numeric signal level (issue #1373) but
narrowed to that case, which is now the only way it can fire.
nicolargo