Implement option to skip null values in symmetric hash impl of stream-stream joins

Implement option to skip null values in symmetric hash impl of stream-stream joins

• In the symmetric has join state manager, we can receive entries with null values for a key and that caused the `removeByValue` and get iterators to fail and run into the NullPointerException.
• This is possible if the state recovered is written from a old spark version or its corrupted on disk. Since we don't have a utility to query this state, we would like to provide a conf option to skip nulls for the symmetric hash impl in stream stream joins.