Fix NRE when custom rules omit optional properties in diagnostics

[rjmholt]

PR Summary

Fixes #1714.

These changes allow custom rules to emit diagnostics (or PSObjects that turn into diagnostics) with optional properties unset.

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[bergmeister]

Looks good, only suggestion is to replace hard-coded property names with code references using things like e.g. nameof. Also, should we try to make the property query case insensitive for the property name itself as I think at the moment it'd be case sensitive