Fix for null parameters

[grubskim]

Null parameter values are changed to an empty string. Tested with the sample controller.

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This change is 

[StavrevaS]

It would be even more secure to add null to empty string transformation into the InstanceParameter.setParameterValue(). The method InstanceParameter.setDecryptedValue() may also set the value, so BEFORE the encryption it should be checked if the value is null and transform it. So in this case the empty string should be encrypted in order to have the same result by decryption.

[grubskim]

Unit tests green, added @StavrevaS suggestion

[grubskim]

Please review and merge if okay.

[StavrevaS]

Reviewed 2 of 2 files at r2. Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[GoebelL]

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Review status: :shipit: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable