OME-TIFF to single channel workflow fails if metadata not contained

**Describe the bug**

Some OME-TIFFs don't have metadata in a format that can be accessed by `tifffile`. In this case, the extractor would fail, since this is used to extract the raw channel names.

**Expected behavior**

If metadata cannot be extracted by `tifffile`, then the extractor should still work. However, the user will need to provide an explicit channel list.

Alternatively, see if there is another OME-TIFF reader that can parse these channel names.

**To Reproduce**

Luke Jones from the University of Birmingham had some example OME-TIFFs that threw an error that the metadata was `None`.