Cannot create streaming pipelines without setting checkpoint directory

https://github.com/cdapio/cdap/pull/11360

Even though it's not in the exported json, the UI is sending an empty string for the checkpointDir to the backend, which causes this issue. Not sure if this was introduced recently or always a problem.

In general, the UI should not be sending empty strings when a value is not provided, as an empty string can possibly mean something different than a null value. In general the backend should also protect against these cases when possible, when an empty string is never an expected valid value.