[test-ot-math] Add test with nil face/font

test/api/test-ot-math.c

diff --git a/test/api/test-ot-math.c b/test/api/test-ot-math.c
index b9fcb65..0ca5566 100644
--- a/test/api/test-ot-math.c
+++ b/test/api/test-ot-math.c
@@ -97,6 +97,14 @@
   g_assert(hb_ot_math_has_data (hb_face)); // MATH table available
   closeFont();
 
+  hb_face = hb_face_get_empty ();
+  hb_font = hb_font_create (hb_face);
+  g_assert(!hb_ot_math_has_data (hb_face)); // MATH table not available
+
+  hb_font = hb_font_get_empty ();
+  hb_face = hb_font_get_face (hb_font);
+  g_assert(!hb_ot_math_has_data (hb_face)); // MATH table not available
+
   cleanupFreeType();
 }