Skip workload mapping when no workloads available

[dvanaken]

For algorithms GPR/DNN, the following error happens when the session has sufficient training data (default is >= 10 samples) but the background task that aggregates the data has not finished running yet:

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/celery/app/trace.py", line 240, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/celery/app/trace.py", line 438, in __protected_call__
    return self.run(*args, **kwargs)
  File "/app/website/website/tasks/async_tasks.py", line 843, in map_workload
    assert len(unique_workloads) > 0
AssertionError

Instead of throwing an exception, we should skip workload mapping and compute the next configuration with the training data from that session.