Calling prov.json before prov.capture

[blernermhc]

If prov.json is called but prov.capture has not been run yet, there is a json string returned but it is not valid. prov.json should probably return NULL in that case so that it is easy for the caller to check this case.

[MKLau]

Yeah, agreed. There should be a NULL message. Something like, "No provenance has been captured yet."

[blernermhc]

I would suggest returning NULL rather than a specific string value. It is much easier to test for NULL.

[MKLau]

Good point, this shouldn't be human human readable anyway. I was thinking from the perspective of cleanR and not provR.