Flatten nulls to empty string for preg_match in PropertyModifierRejectOrSelect

[afoster]

In recent versions of PHP, preg_match() will throw a deprecation notice if null is passed as its $subject string parameter. This change makes matchesRegexp() behave as it would have before when a null was implicitly casted, and is probably what admin users would expect when testing their patterns against a non-existant value versus an empty value.

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[afoster]

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