Minor changes to methods that implement finding wildcard or null in an ast in UseToExportFieldsInManifest.
Minor changes to methods that implement finding wildcard or null in an ast in UseToExportFieldsInManifest.

PowerShell/PSScriptAnalyzer,dlwyatt/PSScriptAnalyzer,daviwil/PSScriptAnalyzer,rkeithhill/PSScriptAnalyzer,juneb/PSScriptAnalyzer