Fix Export-ModuleMember bug for scriptblocks having no context

[PaulHigin]

PR Summary

Export-ModuleMember cmdlet throws an exception if module functions are exported across language boundaries (Windows only). But a scriptblock LanguageMode property can be null if the scriptblock is created without a PowerShell context, and this can happen through the PowerShell API called from C#. In this case Export-ModuleMember throws erroneously when no language mode restrictions are in play.

Fix is to check if LanguageMode is null before comparing context and scriptblock language modes.

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