dask Bag frequencies call generates error for empty partitions

[jspreston]

A call to dask.bag.Bag.frequencies() generates an error if the bag has empty partitions, see the example below:

import dask
import dask.bag as db
import numpy as np

bag = db.from_sequence(np.arange(100), partition_size=10)
freq = bag.filter(lambda x: x < 50).frequencies()
freq.compute()

generates:

IndexError: list index out of range

Traceback
---------
  File "dask/async.py", line 266, in execute_task
    result = _execute_task(task, data)
  File "dask/async.py", line 247, in _execute_task
    return func(*args2)
  File "dask/bag/core.py", line 1651, in empty_safe_aggregate
    return empty_safe_apply(func, parts2)
  File "dask/bag/core.py", line 1646, in empty_safe_apply
    return func(part)
  File "dask/bag/core.py", line 1452, in merge_frequencies
    first, rest = seqs[0], seqs[1:]

[jspreston]

added fix in pull request #1939

[jcrist]

Fixed by #1939. Closing.