Investigate failing parsing YAML on local dev environment

[nicolasbrugneaux]

Context:
Running locally via yarn dev, with a wallet connected to mainnet.

The Govern tab doesn't load, and displays an error instead:

Specifically, CGP 99 triggers the problem.
This proposal has no value (or placeholder) for date-executed. When this proposal is omitted from the list, the page loads without error.

In the function parsedYAMLFromMarkdown defined in src/utils/proposals.ts, it assumes 2-element arrays resulting from splitting a string on a :. For CGP 99, this array only contains a single element, and doesn't fail gracefully in a dev environment.

It seems to work in production.