Update Join serialization test

Author: ivancea

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/plan/logical/JoinSerializationTests.java

@@ -47,9 +47,4 @@ protected Join mutateInstance(Join instance) throws IOException {
         }
         return new Join(instance.source(), left, right, config);
     }
-
-    @Override
-    protected boolean alwaysEmptySource() {
-        return true;
-    }
 }