fix build rule

src/proto/grpc/testing/BUILD

@@ -369,6 +369,15 @@ genrule(
     cmd = "sed 's/messages.proto/messages_gen.proto/' $< > $@",
 )
 
+# The test_gen_proto rule below expects the name that grpc_proto_library
+# would have generated for the dependency, but the dependency was actually
+# generated by a separate proto_library rule, so we use an alias to make
+# things work.
+alias(
+    name = "_empty_proto_only",
+    actual = "empty_proto",
+)
+
 # Consume generated files in srcs and in deps
 grpc_proto_library(
     name = "test_gen_proto",