This is a follow-up to Jens' comment:
19c4b377b8 (commitcomment-23458151)
Now, if there's any axis that has 'interesting' segment maps (and thus
an avar table is added), we also ensure that for the rest of the axes
that aren't modified (either because no <map> elements are defined or
because an identity mapping is defined in the designspace), we always
have a non-empty segment maps array containing the three default maps:
{-1.0: -1.0, 0.0: 0.0, 1.0: 1.0}.
This is to work around CoreText and DirectWrite rendering issue with
empty avar segment maps arrays.