Treat empty message selector as null

[samu-developments]

Ref #439 SQS not supporting non-null message selector.

Probably a better way to solve it, but at least this works for me.

[CLAassistant]

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[samu-developments]

Setting the default to String.class is a better solution as it will set the value to null, https://stackoverflow.com/questions/5389200/what-is-a-java-strings-default-initial-value