* GH-2464: WebFlux: Get rid of Mono.block()
Fixes https://github.com/spring-projects/spring-integration/issues/2464
The `WebFluxInboundEndpoint` resolves a `Principal` via `Mono.block()`
operation.
This is prohibited situation in the non-blocking thread, like Reactor
Netty
* Defer `Mono<Principal>` resolution to the message header via
deferring the whole `Message` creating via `flatMap()` operation on the
main `doHandle()` `Mono`
**Cherry-pick to 5.0.x**
* * Fix Checkstyle for the Reactive Spring Security testing utils static imports
* Add `defaultIfEmpty()`, when `exchange.getPrincipal()` is an empty `Mono`
0fc8bec645