pmb.config.init: Gracefully handle there not being any providrs for _pmb_select

As reported in the merge request removing rproc selection for soc-qcom-msm89161, in the case of _pmb_select being set to a name without providers, pmb init will get stuck on the provider selection step. This is obviously not good as it means the device cannot be selected at all for an unintuitive reason.

So, instead just skip the provider selection and print a warning if there are no providers for a given name. While bugs like this of course should be fixed on a packaging level, let's handle it gracefully as to not completely block people's workflows.