Allow to reset a log level

This commit ensures that `setLogLevel` on the `LoggingSystem` accepts
a `null` level. A `null` level means any customization sets on that
level should be removed and the default configuration should be used
instead.

Effectively, the level of the parent logger is going to be used when
`setLevel` is called with `null` for a given logger.

Most JMX clients do not accept to pass `null` for an argument so an
empty String is translated to null in that specific case.

Closes gh-8776

spring-boot-docs/src/main/asciidoc/production-ready-features.adoc

@@ -927,6 +927,9 @@ In order to configure a given logger, you `POST` a partial entity to the resourc
 	}
 ----
 
+TIP: You can also pass a `null` `configuredLevel` to "reset" the specific level of the
+logger (and use the default configuration instead).
+
 
 
 [[production-ready-metrics]]