Fixed default behaviour of warc handler when no agent is provided

[biosafetylvl5]

Expected/new behaviour:

>>> from archivenow import archivenow
>>> archivenow.push("example.org", "warc", {"warc": "example"})
example.warc
['example.warc']

and example.warc is created.

Current behaviour:

>>> from archivenow import archivenow
>>> archivenow.push("example.org", "warc", {"warc": "example"})
[]

and no file created.