when input passed to rhsda.ApiClient.mget_cves() has no CVEs, return empty list instead of failing

[ryran]

Currently:

>>> o = a.mget_cves("no cves here")
[WARNING] rhsda: No CVEs (matching regex: 'CVE-[0-9]{4}-[0-9]{4,}') found on input
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "rhsda.py", line 670, in mget_cves
    pool = multiprocessing.Pool(processes=numThreads)
  File "/usr/lib64/python2.7/multiprocessing/dummy/__init__.py", line 151, in Pool
    return ThreadPool(processes, initializer, initargs)
  File "/usr/lib64/python2.7/multiprocessing/pool.py", line 727, in __init__
    Pool.__init__(self, processes, initializer, initargs)
  File "/usr/lib64/python2.7/multiprocessing/pool.py", line 152, in __init__
    raise ValueError("Number of processes must be at least 1")
ValueError: Number of processes must be at least 1

[ryran]

After fix:

>>> o = a.mget_cves("no cves here")
[WARNING] rhsda: No CVEs (matching regex: 'CVE-[0-9]{4}-[0-9]{4,}') found on input
>>> o
[]

[ryran]

Actually: should only return an empty list when outFormat='json'; otherwise, should return empty string.