[makeotf] Should not allow writing of empty UVS cmap format 14

[punchcutter]

When reading in a Unicode Variation Sequence mapping file with -ci we get a warning if a CID is not found:

WARNING: Skipping UVS entry for CID '7441': not found in source font.

If every CID in the file is not found we still get an empty cmap format 14 subtable. ots-sanitize then complains with

ERROR: cmap: Bad format 14 subtable records count 0
ERROR: cmap: Failed to parse format 14 cmap subtable 1
ERROR: cmap: Failed to parse table

It would be nice if makeotf dropped the format 14 subtable and gave a warning about it.