TTGlyphPen: do not error with empty contours, simply ignore them

Fixes https://github.com/googlefonts/fontmake/issues/1001

Lib/fontTools/pens/ttGlyphPen.py

@@ -326,7 +326,9 @@ class TTGlyphPointPen(_TTGlyphBasePen, LogMixin, AbstractPointPen):
         if self._isClosed():
             raise PenError("Contour is already closed.")
         if self._currentContourStartIndex == len(self.points):
-            raise PenError("Tried to end an empty contour.")
+            # ignore empty contours
+            self._currentContourStartIndex = None
+            return
 
         contourStart = self.endPts[-1] + 1 if self.endPts else 0
         self.endPts.append(len(self.points) - 1)

Tests/pens/ttGlyphPen_test.py

@@ -353,12 +353,6 @@ class TTGlyphPointPenTest(TTGlyphPenTestBase):
         with pytest.raises(PenError):
             pen.glyph()
 
-    def test_glyph_errorOnEmptyContour(self):
-        pen = TTGlyphPointPen(None)
-        pen.beginPath()
-        with pytest.raises(PenError):
-            pen.endPath()
-
     def test_glyph_decomposes(self):
         componentName = "a"
         glyphSet = {}
@@ -595,6 +589,15 @@ class TTGlyphPointPenTest(TTGlyphPenTestBase):
         assert pen1.points == pen2.points == [(0, 0), (10, 10), (20, 20), (20, 0)]
         assert pen1.types == pen2.types == [1, 1, 0, 1]
 
+    def test_skip_empty_contours(self):
+        pen = TTGlyphPointPen(None)
+        pen.beginPath()
+        pen.endPath()
+        pen.beginPath()
+        pen.endPath()
+        glyph = pen.glyph()
+        assert glyph.numberOfContours == 0
+
 
 class CubicGlyfTest:
     def test_cubic_simple(self):