Handle processes with no inputs and no outputs

[jtniehof]

#86 points out that running DBRunner.py against a process with no inputs and no outputs fails. I'm tagging this both as bug and enhancement: we weren't really planning on supporting this, but the failure mode isn't exactly clean. It's probably easier just to support it.

Relation to an issue

See discussion in #86, also note that this would be affected by #108 (which should probably happen first).

Proposed enhancement

Support running a process with no inputs and no outputs. This would only execute from DBRunner, and wouldn't do anything with the outputs, but it might be handy for someone who needs a regularly-executed process within the dbprocessing framework.

Alternatives

Ensure processes always have at least an input or an output, or at least skip them more cleanly in DBRunner.

OS, Python version, and dependency version information:

Linux-4.15.0-167-generic-x86_64-with-Ubuntu-18.04-bionic
sys.version_info(major=2, minor=7, micro=17, releaselevel='final', serial=0)
sqlalchemy=1.1.11

Version of dbprocessing

Current git (2ca3554)

Closure condition

This issue should be closed when PR merged with DBRunner updates, documentation, and tests for running a process with no inputs/no outputs.