Fix NRE when assigning null to arrayoption in hashtable

[fflaten]

PR Summary

GetArrayOrNull() throws a NRE when value is null, ex. when Run.Path was set to $null in a hashtable. Added null-check to return before any use of .GetType() etc.

Fixing this hashtable-scenario because Pester ignores null-values while casting from hashtable and the error didn't make it clear which option had the problem:

InvalidArgument: Cannot convert value "System.Collections.Hashtable" to type "PesterConfiguration". Error: "Object reference not set to an instance of an object."

Fix #2026

PR Checklist

• PR has meaningful title
• Summary describes changes
• PR is ready to be merged
  • If not, use the arrow next to Create Pull Request to mark it as a draft. PR can be marked Ready for review when it's ready.
• Tests are added/update (if required)
• Documentation is updated/added (if required)

[fflaten]

There's a difference between assigning null in a hashtable and directly to the options in a PesterConfiguration-object. During casting from hashtable, null-values are ignored in general. Assigning to options in a configuration-object allows null in most cases, except for booloptions which throws an NRE.

Should we check for null in direct assignments in advanced config in general so both hashtables and direct assignment behave equally? Ex. null-valued paths, scriptblocks etc. are only going to cause issues later on.