Improve validation for AsyncWaitStrategyFactoryConfig for null/empty 'factoryClassName'. #3159

[JWT007]

This should fix a potential NPE in AsyncWaitStrategyFactoryConfig if the builder 'class' field is not set or set to null/empty string. #3159

• added notEmpty assert to Builder setFactoryClassName
• changed notNull to notEmpty assert to AsyncWaitStrategyFactoryConfig constructor.
• added changelog XML file.
• added new tests to AsyncWaitStrategyFactoryConfigTest

Checklist

• Base your changes on 2.x branch if you are targeting Log4j 2; use main otherwise
• ./mvnw verify succeeds (if it fails due to code formatting issues reported by Spotless, simply run ./mvnw spotless:apply and retry)
• Non-trivial changes contain an entry file in the src/changelog/.2.x.x directory
• Tests for the changes are provided
• Commits are signed (optional, but highly recommended)

[github-actions]

Job	Requested goals	Build Tool Version	Build Outcome	Build Scan®
build-macos-latest	clean install	3.9.8	❌
build-ubuntu-latest	clean install	3.9.8	❌
build-windows-latest	clean install	3.9.8	❌

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