Gracefully handle empty stanzas in YAML run description

**[Original report](https://bitbucket.org/salishsea/nemo-cmd/issue/12) by Doug Latornell (Bitbucket: [douglatornell](https://bitbucket.org/douglatornell), GitHub: [douglatornell](https://github.com/douglatornell)).**

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Known problem stanzas:

* `forcing:`
* `restart:`
* `vcs revisions: hg:`
