Don't add empty glyphs without painted layers to COLR or SVG table

as found out in https://github.com/fonttools/fonttools/pull/2441/files#r744692672

src/nanoemoji/svg.py

@@ -652,7 +652,8 @@ def _picosvg_docs(
         svg = SVG(root)
 
         for color_glyph in (color_glyphs[g] for g in group):
-            _add_glyph(svg, color_glyph, reuse_cache)
+            if color_glyph.painted_layers:
+                _add_glyph(svg, color_glyph, reuse_cache)
 
         # tidy use elements, they may emerge from _add_glyph with unnecessary attributes
         _tidy_use_elements(svg)
@@ -664,6 +665,9 @@ def _picosvg_docs(
         if len(defs) == 0:
             root.remove(defs)
 
+        if len(root) == 0:
+            continue
+
         gids = tuple(color_glyphs[g].glyph_id for g in group)
         doc_list.append(
             (svg.tostring(pretty_print=config.pretty_print), min(gids), max(gids))

src/nanoemoji/write_font.py

@@ -592,10 +592,11 @@ def _colr_ufo(
             color_glyph, glyph_cache
         )
 
-        # write out the ufo structures for COLR
-        ufo_color_layers[color_glyph.glyph_name] = _ufo_colr_layers(
-            colr_version, colors, color_glyph
-        )
+        if color_glyph.painted_layers:
+            # write out the ufo structures for COLR
+            ufo_color_layers[color_glyph.glyph_name] = _ufo_colr_layers(
+                colr_version, colors, color_glyph
+            )
         bounds = _bounds(color_glyph, quantization)
         if bounds is not None:
             clipBoxes.setdefault(bounds, []).append(color_glyph.glyph_name)

tests/nanoemoji_test.py

@@ -178,3 +178,26 @@ def test_glyphmap_games():
     assert_expected_ttx(
         svgs, font, "glyphmap_games.ttx", include_tables=["GlyphOrder", "cmap"]
     )
+
+
+def test_omit_empty_color_glyphs():
+    svgs = [
+        "emoji_u200c.svg",  # whitespace glyph, contains no paths
+        "emoji_u42.svg",
+    ]
+
+    tmp_dir = _run(
+        (
+            "--color_format=glyf_colr_1_and_picosvgz",
+            "--keep_glyph_names",
+            *(locate_test_file(svg) for svg in svgs),
+        )
+    )
+
+    font = TTFont(tmp_dir / "Font.ttf")
+
+    colr = font["COLR"].table
+    assert len(colr.BaseGlyphList.BaseGlyphPaintRecord) == 1
+
+    svg = font["SVG "]
+    assert len(svg.docList) == 1