algo fixed does not run when there are no systematics minimized by Minuit

[adewit]

See: https://cms-talk.web.cern.ch/t/nll-at-fixed-poi/19124/1 - specific issue is a setup with only autoMCStats as an uncertainty, and one POI, so that (with analytic minimization on by default) Minuit is presented with a model that has no floating parameters.

We should catch this case in MultiDimFit and instead of calling minimize, simply evaluate the likelihood at the given fixed point.

Not fixing immediately as this situation is not likely to occur frequently.