Log4j 3.x - StringMatchFilter - guard potential NPEs and builder validation/API

[JWT007]

This is a parallel of Ticket #3153 for the main (3.x) branch.

StringMatchFilter:

• add validation on 'text' attribute to class constructor and builder (non-blank)
• add JVerify nullability annotations to class
• replace 'setMatchString' builder method with 'setText'
• add javadoc details about the specific StringMatchFilter implementation/behaviour
• update tests