[outlineOTF] only write PostScript BlueValues when are not empty

[anthrotype]

Currently ufo2ft writes all the four blue values (BlueValues, OtherBlues, FamilyBlues, FamilyOtherBlues) whenever either postscriptBlueValues or postscriptOtherBlues are non-empty lists in the UFO fontinfo.plist. It only checks those two, but then writes all four of them.

So even if the others are empty, they will still be compiled as empty deltas in the CFF PrivateDict.

Now, it turns out the OpenType Sanitizer will reject such CFF fonts with empty Blues operators in the PrivateDict:

See https://github.com/khaledhosny/ots/blob/dcfa05f/src/cff.cc#L378-L385

I am not sure whether it's OTS that is being too picky, or whether these empty deltas in PrivateDict are actually invalid in CFF.

In any case, we can work around this OTS failure by setting each of these PrivateDict blues only if they are not empty -- rather than checking if only two of them are non-empty.

The same ufo2ft behaviour of writing empty deltas was also triggering another fonttools issue (now fixed):
fonttools/fonttools#883

[anthrotype]

/cc @miguelsousa

[miguelsousa]

I think OTS is being too picky, but I this is a good change, so 👍

[anthrotype]

Thanks Miguel. I'm cc-ing @khaledhosny who maintains OTS to see if we could relax that check a bit.
From my reading of the CFF PrivateDict spec, those operators appear to be all optional, but also there is no mention that if they are there they must contain non-empty values.
The only mention about one of them being required is BlueValues, but that was the Type 1 spec.

[khaledhosny]

I think it is safer (and quicker) to just not write empty operator than for me to try to guess what the original OTS author(s) had in mind when this check was written, fix it, then wait for the new version of OTS to propagate to the browsers and for significant users to switch to new enough versions. It will probably be several years, at best, before one can rely on this anyway. :)

But feel free to open an issue against OTS so that we can look into this at some point.