[subset] MATH: don't serialize coverage table when iterator is empty

when iterator is empty, just set coverage offset to 0.
serialize() in coverage will at lease write out a 16-bit format header.

src/hb-ot-math-table.hh

@@ -511,7 +511,8 @@ struct MathGlyphInfo
     | hb_map_retains_sorting (glyph_map)
     ;
 
-    out->extendedShapeCoverage.serialize_serialize (c->serializer, it);
+    if (it) out->extendedShapeCoverage.serialize_serialize (c->serializer, it);
+    else out->extendedShapeCoverage = 0;
 
     out->mathKernInfo.serialize_subset (c, mathKernInfo, this);
     return_trace (true);
@@ -884,8 +885,11 @@ struct MathVariants
       if (!o) return_trace (false);
       o->serialize_subset (c, glyphConstruction[i], this);
     }
-
-    out->vertGlyphCoverage.serialize_serialize (c->serializer, new_vert_coverage.iter ());
+    
+    if (new_vert_coverage)
+      out->vertGlyphCoverage.serialize_serialize (c->serializer, new_vert_coverage.iter ());
+    
+    if (new_hori_coverage)
     out->horizGlyphCoverage.serialize_serialize (c->serializer, new_hori_coverage.iter ());
     return_trace (true);
   }

test/subset/data/Makefile.am

@@ -52,6 +52,7 @@ EXTRA_DIST += \
 	expected/variable \
 	expected/glyph_names \
 	expected/math \
+	expected/math_coverage_offset \
 	expected/post \
 	fonts \
 	profiles \

test/subset/data/Makefile.sources

@@ -44,6 +44,7 @@ TESTS = \
 	tests/variable.tests \
 	tests/glyph_names.tests \
 	tests/math.tests \
+	tests/math_coverage_offset.tests \
 	tests/post.tests \
 	$(NULL)
 

test/subset/data/tests/math_coverage_offset.tests

@@ -0,0 +1,12 @@
+FONTS:
+Caudex-Regular.ttf
+
+PROFILES:
+default.txt
+retain-gids.txt
+glyph-names.txt
+notdef-outline.txt
+keep-all-layout-features.txt
+
+SUBSETS:
+*

test/subset/meson.build

@@ -39,6 +39,7 @@ tests = [
   'sbix',
   'colr',
   'math',
+  'math_coverage_offset',
 # TODO: re-enable once colrv1 subsetting is stabilized.
 # 'colrv1.notoemoji',
   'colrv1',