`as.matrix(Cmcmc$mvSamples)` fails if nothing was monitored

[perrydv]

See nimble-users message of 7/9/21.

[paciorek]

Side note: Setting nothing to be monitored seems to require either a corner case such as the user gave or use of resetMonitors as using monitors = NULL in configureMCMC seems to have no effect. I'm going to fix things so monitors = NULL "works", though it's not useful, presumably.

[paciorek]

I made a variety of adjustments for the no monitors case. Probably didn't catch every possible error and what is output (NULL versus empty list, etc.) may not be completely consistent across various ways to run and report MCMC, but hopefully handles most things ok.

Also, the default value for missing modelValues values is now as.numeric(NA) instead of 0.1. I think this might only affect uncompiled modelValues.

As far as monitors = NULL that in essence is the default, which means monitor top nodes. So I will just leave as is, such that asking for monitors = NULL gives top nodes as well.

[paciorek]

Fixed by PR #1153.