GF Core filter list - NULL glyph name

The default Glyphs GlyphData.xml file defines the glyph name `.null` rather than `NULL`:


According to the Adobe AGL/OpenType Feature File specs, `.notdef` is the only name that should include a leading period:

> ...must be entirely comprised of characters from the following set:
>
>A-Z a-z 0-9 . (period) _ (underscore)
>
>and must not start with a digit or period. The only exception is the special character ".notdef".

My read of the specs leads me to believe that this applies to "development names" as well.

The name that Glyphs uses in the GlyphData.xml file appears to be an invalid name according to those specifications. Does this warrant a change in the GF Core filter list glyph name definitions, a change in the GlyphData.xml file, no action? Does NULL act as a proper glyph name for Glyphs filters if it is not mapped to this nice name in the GlyphData.xml file?

Start by comparing the `.null` entry in the default Glyphs GlyphData.xml file with the GF Core filter list glyph name definitions and the cited Adobe AGL/OpenType Feature File specifications. Resolve whether development names are covered, then document whether the fix belongs in GlyphData.xml, the filter list, or neither; the issue is done when that decision and its effect on Glyphs filters are established.

Written by the indexing model from the issue text.