xFirefoxPreference: Error is thrown if preconfig file doesn't exist

[chasewilson]

Details of the scenario you tried and the problem that is occurring

Running the firefox preference resource. if the preconfig file doesn't exist and error is thrown due to an undefined variable.

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The DSC configuration that is used to reproduce the issue (as detailed as possible)

# insert configuration here

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Version of the DSC module that was used ('dev' if using current dev branch)