FIX: Missing coordinates.xml in MFF file

[scott-huberty]

The EGI MFF reader already tries to account for the case where a sensor coordinate file (coordinates.xml) is missing:

mne-python/mne/io/egi/egimff.py

Lines 275 to 284 in a5073bf

	def _read_locs(filepath, egi_info, channel_naming):
	"""Read channel locations."""
	_soft_import("defusedxml", "reading EGI MFF data")
	from defusedxml.minidom import parse
	fname = op.join(filepath, "coordinates.xml")
	if not op.exists(fname):
	logger.warn("File coordinates.xml not found, not setting channel locations")
	ch_names = [channel_naming % (i + 1) for i in range(egi_info["n_channels"])]
	return ch_names, None

So if this variable mon can be None, then all the code dealing with it should be nested inside the if-block below:

mne-python/mne/io/egi/egimff.py

Line 449 in a5073bf

ch_names, mon = _read_locs(input_fname, egi_info, channel_naming)

mne-python/mne/io/egi/egimff.py

Lines 488 to 491 in a5073bf

	if mon is not None:
	info.set_montage(mon, on_missing="ignore")
	ref_idx = np.flatnonzero(np.isin(mon.ch_names, REFERENCE_NAMES))

I also updated a test to simulate this case.

[larsoner]

Looks good, marking for merge-when-green, thanks @scott-huberty !