…xception: Could not load key store 'null'" / "IllegalArgumentException: Resource location must not be null" when using PKCS11 keystore type ('server.ssl.key-store-type') with undefined/null 'server.ssl.key-store' property (as there is no keystore file to be specified for a PKCS#11 HSM) on a Spring Boot Reactor Netty configuration.

- Added unit tests in spring-boot-project/spring-boot SslServerCustomizerTests class with a mock PKCS#11 keystore provider implementation to validate the fix
- Added TestContainers integration test loading a Spring Boot Webflux (Netty SSL server) app with an actual PKCS#11 HSM (SoftHSM) for the keystore: to make sure it works with real implementation: see project spring-boot-tests/spring-boot-smoke-tests/spring-boot-test-webflux-ssl

…for Netty previously. This is part of PR spring-projects#32179.

…izer classes