Fix CMake error on CE build

Summary:
D4953 introduced a break in the build. Building community would fail with:
```
08:28:24 CMake Error at src/yb/rpc/CMakeLists.txt:144 (ADD_YB_TESTS):
08:28:24   ADD_YB_TESTS Function invoked with incorrect arguments for function named:
08:28:24   ADD_YB_TESTS
```

Test Plan: try build with --edition community

Reviewers: sergei, kannan

Reviewed By: kannan

Subscribers: ybase

Differential Revision: https://phabricator.dev.yugabyte.com/D4999

src/yb/rpc/CMakeLists.txt

@@ -141,4 +141,6 @@ ADD_YB_TEST(rpc-test)
 ADD_YB_TEST(rpc_stub-test RUN_SERIAL true)
 ADD_YB_TEST(scheduler-test)
 ADD_YB_TEST(thread_pool-test)
-ADD_YB_TESTS(${RPC_ADDITIONAL_TESTS})
+if(RPC_ADDITIONAL_TESTS)
+  ADD_YB_TESTS(${RPC_ADDITIONAL_TESTS})
+endif()