[regexp] Handle unmatched groups in callable replacers

src/runtime/runtime-regexp.cc

Index: src/runtime/runtime-regexp.cc
diff --git a/src/runtime/runtime-regexp.cc b/src/runtime/runtime-regexp.cc
index e5d6e4c71ed8cfac85fc4d823d1cb45c89bf1a80..0bc874aa86205cf2071cf5c585706126c8ea80ac 100644
--- a/src/runtime/runtime-regexp.cc
+++ b/src/runtime/runtime-regexp.cc
@@ -1125,7 +1125,7 @@ Handle<JSObject> ConstructNamedCaptureGroupsObject(
     DCHECK(1 <= capture_ix && capture_ix <= capture_count);
 
     Handle<Object> capture_value(f_get_capture(capture_ix), isolate);
-    DCHECK(capture_value->IsString());
+    DCHECK(capture_value->IsUndefined(isolate) || capture_value->IsString());
 
     JSObject::AddProperty(groups, capture_name, capture_value, NONE);
   }