Fix "gn gen" when using ios_framework_bundle with no public_headers

A framework bundle does not require public headers, so change the
ios_framework_bundle template to work if public_headers is empty or
undefined.

Bug: none
Change-Id: Ieac39fd98220d23f5ae6bf99908d19112ef2eb0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1944476
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720513}

build/config/ios/rules.gni

@@ -1219,10 +1219,11 @@ template("ios_framework_bundle") {
 
   _default_toolchain_root_out_dir =
       get_label_info("$_target_name($default_toolchain)", "root_out_dir")
-  _default_toolchain_target_gen_dir =
-      get_label_info("$_target_name($default_toolchain)", "target_gen_dir")
 
   if (_has_public_headers) {
+    _default_toolchain_target_gen_dir =
+        get_label_info("$_target_name($default_toolchain)", "target_gen_dir")
+
     _framework_headers_target = _target_name + "_framework_headers"
     _framework_headers_config = _target_name + "_framework_headers_config"
     config(_framework_headers_config) {