[draw/glyf] Emit empty contours

This makes it actually match freetype behaviour even though rasterizer
should filter such contours specially for stroking.

See https://github.com/harfbuzz/harfbuzz/pull/3411#discussion_r802283827 for the context.

src/hb-ot-glyf-table.hh

@@ -1251,6 +1251,12 @@ struct glyf
 				       font->em_fscalef_x (first_oncurve.x), font->em_fscalef_y (first_oncurve.y));
 	  else if (first_oncurve.has_data)
 	    draw_session->line_to (font->em_fscalef_x (first_oncurve.x), font->em_fscalef_y (first_oncurve.y));
+	  else if (first_offcurve.has_data)
+	  {
+	    float x = font->em_fscalef_x (first_offcurve.x), y = font->em_fscalef_x (first_offcurve.y);
+	    draw_session->move_to (x, y);
+	    draw_session->quadratic_to (x, y, x, y);
+	  }
 
 	  /* Getting ready for the next contour */
 	  first_oncurve = first_offcurve = last_offcurve = optional_point_t ();

test/api/test-draw.c

@@ -369,7 +369,7 @@ test_hb_draw_ttf_parser_tests (void)
     {
       draw_data.consumed = 0;
       hb_font_get_glyph_shape (font, 5, funcs, &draw_data);
-      char expected[] = "";
+      char expected[] = "M15,0Q15,0 15,0Z";
       g_assert_cmpmem (str, draw_data.consumed, expected, sizeof (expected) - 1);
     }
     {
@@ -378,7 +378,8 @@ test_hb_draw_ttf_parser_tests (void)
       char expected[] = "M346,468L211,468L211,0L123,0L123,468L29,468L29,509L123,539"
 			"L123,570Q123,674 169,720Q215,765 297,765Q329,765 356,760"
 			"Q382,754 401,747L378,678Q362,683 341,688Q320,693 298,693"
-			"Q254,693 233,664Q211,634 211,571L211,536L346,536L346,468Z";
+			"Q254,693 233,664Q211,634 211,571L211,536L346,536L346,468Z"
+			"M15,0Q15,0 15,0Z";
       g_assert_cmpmem (str, draw_data.consumed, expected, sizeof (expected) - 1);
     }