[subset] Cmap table: remove encodingRecord entry for empty cmap4 subtable

src/hb-ot-cmap-table.hh

diff --git a/src/hb-ot-cmap-table.hh b/src/hb-ot-cmap-table.hh
index 13e8b3c..38dca26 100644
--- a/src/hb-ot-cmap-table.hh
+++ b/src/hb-ot-cmap-table.hh
@@ -238,16 +238,18 @@
   void serialize (hb_serialize_context_t *c,
 		  Iterator it)
   {
-    unsigned table_initpos = c->length ();
-    if (unlikely (!c->extend_min (*this))) return;
-    this->format = 4;
-
     auto format4_iter =
     + it
     | hb_filter ([&] (const hb_pair_t<hb_codepoint_t, hb_codepoint_t> _)
 		 { return _.first <= 0xFFFF; })
     ;
 
+    if (format4_iter.len () == 0) return;
+
+    unsigned table_initpos = c->length ();
+    if (unlikely (!c->extend_min (*this))) return;
+    this->format = 4;
+
     //serialize endCode[]
     HBUINT16 *endCode = serialize_endcode_array (c, format4_iter);
     if (unlikely (!endCode)) return;

test/api/fonts/Roboto-Regular.empty.ttf

diff --git a/test/api/fonts/Roboto-Regular.empty.ttf b/test/api/fonts/Roboto-Regular.empty.ttf
new file mode 100644
index 0000000..fbd4fba
--- /dev/null
+++ b/test/api/fonts/Roboto-Regular.empty.ttf
Binary files differ

test/api/test-subset-cmap.c

diff --git a/test/api/test-subset-cmap.c b/test/api/test-subset-cmap.c
index f9f8f54..a6c24fc 100644
--- a/test/api/test-subset-cmap.c
+++ b/test/api/test-subset-cmap.c
@@ -113,6 +113,26 @@
   hb_face_destroy (face_origin);
 }
 
+static void
+test_subset_cmap_empty_tables (void)
+{
+  hb_face_t *face_abc = hb_test_open_font_file ("fonts/Roboto-Regular.abc.ttf");
+  hb_face_t *face_empty = hb_test_open_font_file ("fonts/Roboto-Regular.empty.ttf");
+
+  hb_set_t *codepoints = hb_set_create ();
+  hb_face_t *face_abc_subset;
+  hb_set_add (codepoints, 100);
+  hb_set_add (codepoints, 101);
+  face_abc_subset = hb_subset_test_create_subset (face_abc, hb_subset_test_create_input (codepoints));
+  hb_set_destroy (codepoints);
+
+  hb_subset_test_check (face_empty, face_abc_subset, HB_TAG ('c','m','a','p'));
+
+  hb_face_destroy (face_abc_subset);
+  hb_face_destroy (face_abc);
+  hb_face_destroy (face_empty);
+}
+
 // TODO(rsheeter) test cmap to no codepoints
 
 int
@@ -124,6 +144,7 @@
   hb_test_add (test_subset_cmap_noop);
   hb_test_add (test_subset_cmap_non_consecutive_glyphs);
   hb_test_add (test_subset_cmap4_no_exceeding_maximum_codepoint);
+  hb_test_add (test_subset_cmap_empty_tables);
 
   return hb_test_run();
 }

test/subset/data/expected/japanese/Mplus1p-Regular.default.25771.ttf

diff --git a/test/subset/data/expected/japanese/Mplus1p-Regular.default.25771.ttf b/test/subset/data/expected/japanese/Mplus1p-Regular.default.25771.ttf
new file mode 100644
index 0000000..423b406
--- /dev/null
+++ b/test/subset/data/expected/japanese/Mplus1p-Regular.default.25771.ttf
Binary files differ

test/subset/data/expected/japanese/Mplus1p-Regular.drop-hints.25771.ttf

diff --git a/test/subset/data/expected/japanese/Mplus1p-Regular.drop-hints.25771.ttf b/test/subset/data/expected/japanese/Mplus1p-Regular.drop-hints.25771.ttf
new file mode 100644
index 0000000..d735936
--- /dev/null
+++ b/test/subset/data/expected/japanese/Mplus1p-Regular.drop-hints.25771.ttf
Binary files differ

test/subset/data/tests/japanese.tests

diff --git a/test/subset/data/tests/japanese.tests b/test/subset/data/tests/japanese.tests
index b4fe387..d923491 100644
--- a/test/subset/data/tests/japanese.tests
+++ b/test/subset/data/tests/japanese.tests
@@ -12,5 +12,6 @@
 あいうえおか
 あいう珠度輸
 𝜕𝟘AB
+𥝱