Add an error for empty script names in GN.

tools/gn/action_target_generator.cc

Index: tools/gn/action_target_generator.cc
diff --git a/tools/gn/action_target_generator.cc b/tools/gn/action_target_generator.cc
index c51910c61b03752a7774e7dfa21627565dd95b1d..12b621ea4f12108d4128e4b66427d9eb8ae57b43 100644
--- a/tools/gn/action_target_generator.cc
+++ b/tools/gn/action_target_generator.cc
@@ -94,8 +94,13 @@ void ActionTargetGenerator::FillScript() {
   if (!value->VerifyTypeIs(Value::STRING, err_))
     return;
 
-  target_->action_values().set_script(
-      scope_->GetSourceDir().ResolveRelativeFile(value->string_value()));
+  SourceFile script_file =
+      scope_->GetSourceDir().ResolveRelativeFile(value->string_value());
+  if (script_file.value().empty()) {
+    *err_ = Err(*value, "script name is empty");
+    return;
+  }
+  target_->action_values().set_script(script_file);
 }
 
 void ActionTargetGenerator::FillScriptArgs() {