UnboundedSourceWrapper checkpoints before starting Readers

When start has not been called on the Readers of CountingSource, the checkpoint may contain a null value which lets the AvroCoder fail with a NullpointerException.

UnboundedSourceWrapper should not attempt to checkpoint before the readers have been started. See UnboundedSource:

> getCheckpointMark not be called before \{@link #start\} has been called.

Imported from Jira [BEAM-6315](https://issues.apache.org/jira/browse/BEAM-6315). Original Jira may contain additional context.
Reported by: mxm.

Start with UnboundedSourceWrapper and the CountingSource readers, then inspect how checkpointing is triggered before reader start. Verify that checkpointing is skipped until readers have started and that the null checkpoint no longer causes the AvroCoder NullPointerException.

Written by the indexing model from the issue text.