[SPARK-14455][STREAMING] Fix NPE in allocatedExecutors when calling in receiver-less scenario

## What changes were proposed in this pull request?

When calling `ReceiverTracker#allocatedExecutors` in receiver-less scenario, NPE will be thrown, since this `ReceiverTracker` actually is not started and `endpoint` is not created.

This will be happened when playing streaming dynamic allocation with direct Kafka.

## How was this patch tested?

Local integrated test is done.

Author: jerryshao <sshao@hortonworks.com>

Closes #12236 from jerryshao/SPARK-14455.