Fix handling of empty metadata when uploading data files

[shoeffner]

Relates to datalad/datalad-dataverse#320: Handles "empty" metadata and is an alternative to datalad/datalad-dataverse#322.

[pdurbin]

At a high level, this seems like a good fix. The JSON should be optional.

[JR-1991]

LGTM - Thanks!

[adswa]

many thanks again for this fix :) Is there by chance a pyDataverse release upcoming? Otherwise, I'll let datalad-dataverse depend on a development version from github for the time being. :)

[pdurbin]

@adswa hi! I'll defer to @JR-1991 on the timing of the release but it should be 0.3.4. I just assigned that milestone to this PR (which was a PR to #203 which also has that milestone).

I don't see any problem with using a dev version, in the meantime.

[JR-1991]

@adswa - Once the final two pull requests are merged and all issues are resolved within the milestone, we will be all set to release the next version 0.3.4 of pyDataverse. Both are relatively small and I am confident that we will have the release out very soon.