"Rename Glyphs" causes crash in glyphsLib.builder.instances

[simoncozens]

I'm trying to build a variable font setting which has a Rename Glyphs custom parameter. This causes us to go down the _to_designspace_varfont code path, which does this:

glyphsLib/Lib/glyphsLib/builder/instances.py

Lines 93 to 95 in 050ef62

	# to_ufo_custom_params() wants a UFO, create a dummy one
	ufo = self.ufo_module.Font()
	to_ufo_custom_params(self, ufo, instance, set_default_params=False)

When processing custom parameters on this new, empty UFO, glyphsLib tries to process the Rename Glyphs custom parameter by swapping the glyph assignments around, but of course there are no glyphs in the font, so we get a KeyError and a crash:

 line 95, in _to_designspace_varfont
    to_ufo_custom_params(self, ufo, instance, set_default_params=False)
  File "/Users/simon/others-repos/glyphsLib/Lib/glyphsLib/builder/custom_params.py", line 1112, in to_ufo_custom_params
    handler.to_ufo(self, glyphs_proxy, ufo_proxy)
  File "/Users/simon/others-repos/glyphsLib/Lib/glyphsLib/builder/custom_params.py", line 1072, in to_ufo
    ufo[newname], ufo[oldname] = ufo[oldname], ufo[newname]
                                 ~~~^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/ufoLib2/objects/font.py", line 240, in __getitem__
    return self.layers._defaultLayer[name]
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/ufoLib2/objects/layer.py", line 189, in __getitem__
    glyph_object = self._glyphs[name]
                   ~~~~~~~~~~~~^^^^^^
KeyError: 'A.ss02'