Do not create empty guidelines array

[frankrolf]

Before this change, any f.save() would create an empty guidelines array in the fontinfo.plist.
Since guidelines are an optinal attribute, I do not think this is necessary.

[typemytype]

this feels very dangerous.. the attribute font.guidelines is a list or an Iterator.

I assume you dont want empty arrays in the ufo xml, this should be moved to fontTools and ufoLib.

[frankrolf]

Yes, I can see that some tests are failing so possibly not the best fix here – do you have a suggestion where else it should go?

[typemytype]

ufoLib writes the font.info object, defcon mirrors font.guidelines in the font info object: see https://github.com/robotools/defcon/blob/master/Lib/defcon/objects/info.py#L220-L232

It could be fine to make the info.guidelines more ufoLib friendly and return None if the list is empty. The doc string already clarifies that this attribute is just to have a ufoLib compatibility attribute.

@typesupply

[typesupply]

This sounds fine to me.