use empty .notdef for sparse TTF masters

previously we were copying one from the default master, but if this contains cubic curves our own check fails because the .notdef we inserted in the sparse masters OutlineCompiler was not passed through cu2qu...
Our intention is to have sparse masters not participate in the interpolation of that particular glyph so we make it empty (gid0=.notdef still needs to be there in a valid TTF so we must have one).
Currently the same trick does not work for CFF2 variable fonts (fonttools/fonttools#3233) but those are fine with cubics in .notdef glyph any way.. For now at least

Lib/ufo2ft/__init__.py

@@ -408,7 +408,7 @@ def compileInterpolatableTTFsFromDS(designSpaceDoc, **kwargs):
     kwargs["skipExportGlyphs"] = designSpaceDoc.lib.get("public.skipExportGlyphs", [])
 
     if kwargs["notdefGlyph"] is None:
-        kwargs["notdefGlyph"] = _getDefaultNotdefGlyph(designSpaceDoc)
+        kwargs["notdefGlyph"] = _getDefaultNotdefGlyph(designSpaceDoc, empty=True)
 
     kwargs["extraSubstitutions"] = defaultdict(set)
     for rule in designSpaceDoc.rules:

Lib/ufo2ft/util.py

@@ -475,7 +475,7 @@ def getDefaultMasterFont(designSpaceDoc):
     return defaultSource.font
 
 
-def _getDefaultNotdefGlyph(designSpaceDoc):
+def _getDefaultNotdefGlyph(designSpaceDoc, empty=False):
     from ufo2ft.errors import InvalidDesignSpaceData
 
     try:
@@ -488,6 +488,18 @@ def _getDefaultNotdefGlyph(designSpaceDoc):
             notdefGlyph = baseUfo[".notdef"]
         except KeyError:
             notdefGlyph = None
+        else:
+            if empty:
+                # create a new empty .notdef glyph with the same width/height
+                # as the default master's .notdef glyph to be use for sparse layer
+                # master TTFs, so that it won't participate in gvar interpolation
+                # TODO(anthrotype): Use sentinel values for width/height to
+                # mark the glyph as non-participating for HVAR if/when fonttools
+                # supports that, https://github.com/googlefonts/ufo2ft/issues/501
+                emptyGlyph = _getNewGlyphFactory(notdefGlyph)(".notdef")
+                emptyGlyph.width = notdefGlyph.width
+                emptyGlyph.height = notdefGlyph.height
+                notdefGlyph = emptyGlyph
     return notdefGlyph