Fix NullReferenceException in AvoidAssignmentToAutomaticVariable rule when assigning a .Net property and only look at the LHS

[bergmeister]

PR Summary

• Fixes NullReferenceException in PSAvoidAssignmentToAutomaticVariable rule when assigning .Net property to .Net property (v 1.17.0) #1007 by adding a null check for the variable variableExpressionAst
• Fixes PSAvoidAssignmentToAutomaticVariable finds auto variable in RHS when assigning to a static property #1013 by only looking at the LHS for variable names.
• Fixes PSAvoidAssignmentToAutomaticVariable triggers on property assignment of automatic variable #1012 by making sure the VariableExpressionAst's parent is the AssignmentAst (i.e. not a property of the variable or something like that)

The issues are slightly related to each other, therefore they are both addressed in this PR to keep things simple.

PR Checklist

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[bergmeister]

only this null check was added, all the code diff below is only the added indentation

[JamesWTruher]

from a flow perspective, this could also be

if ( variableExpressionAst == null )
   continue

and the indenting is preserved :)

[JamesWTruher]

neither of my comments are blockers

[JamesWTruher]

shouldn't this just be:
{ Invoke-ScriptAnalyzer -ScriptDefinition '[foo]::bar = [baz]::quz' -ErrorAction Stop } | Should Not Throw
It will make the log easier to understand if it fails.

[bergmeister]

Pester only works with terminating errors. PSSA throws non-terminating errors, see pester/Pester#366

[JamesWTruher]

i would have thought that -ErrorAction Stop would do the trick (after all, it's in a try/catch, so you're doing the same thing) so somebody is halting the pipeline

[JamesWTruher]

from a flow perspective, this could also be

if ( variableExpressionAst == null )
   continue

and the indenting is preserved :)

[JamesWTruher]

lgtm

[JamesWTruher]

i would have thought that -ErrorAction Stop would do the trick (after all, it's in a try/catch, so you're doing the same thing) so somebody is halting the pipeline