fix get_supported_channels for single-channel instruments

Instruments without `channels:` in config.yaml (HERMES, and the
defaults base) had get_supported_channels() returning None, causing
test_channelinfo / test_sort_files to fail on `for channel in None`.
CRIRES_PLUS and METIS_IFU avoided it by overriding the method;
the base class now falls back to [None] so the single-channel case
iterates once with channel=None.

Author: ivh

pyreduce/instruments/common.py

@@ -892,7 +892,9 @@ def get_wavecal_filename(self, header, channel, **kwargs):
         return fname
 
     def get_supported_channels(self):
-        return self.channels
+        # Single-channel instruments with no `channels:` in config.yaml
+        # iterate once with channel=None.
+        return self.channels or [None]
 
     def get_settings_fallbacks(self, channel):
         """Return channel names to try when looking up settings files.