Add parameter validation in RegexNameRewriter and RegexNameValidatingRewriter to ensure the provided substitution pattern is not null. The Javadoc for the parameter should be updated as well to indicate that it must not be null.

An example of how to add a check to make sure a parameter is not null can be found here:
https://github.com/wildfly-security/wildfly-elytron/blob/0bc21f17dab0507f7a5cdf7d2e7ea14d44eae12a/credential/base/src/main/java/org/wildfly/security/credential/PasswordCredential.java#L51