Protect against NPE caused by recursive calls

Update `SpringBootConfigurationFactory` so that it no longer attempts
to get a `LoggingSystem`.

The recent `LoggingSystem` update means makes use of the
`SpringFactoriesLoader` class to load candidate logging systems.
Unfortunately, the `SpringFactoriesLoader` class creates a `Logger`
which (when using Log4J2) causes `SpringBootConfigurationFactory` to
run. Calling `LoggingSystem.get` from `SpringBootConfigurationFactory`
results in a recursive call to `SpringFactoriesLoader` which hasn't
yet been fully initialized. We then see an NPE caused by a `null`
`cache`.

This update removes the call to `LoggingSystem.get` with the assumption
that it would never return `null` anyway.

Fixes gh-24163

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/SpringBootConfigurationFactory.java

@@ -24,8 +24,6 @@ import org.apache.logging.log4j.core.config.DefaultConfiguration;
 import org.apache.logging.log4j.core.config.Order;
 import org.apache.logging.log4j.core.config.plugins.Plugin;
 
-import org.springframework.boot.logging.LoggingSystem;
-
 /**
  * Spring Boot {@link ConfigurationFactory} that customizes Log4J2's default configuration
  * to:
@@ -56,12 +54,11 @@ public class SpringBootConfigurationFactory extends ConfigurationFactory {
 
 	@Override
 	public Configuration getConfiguration(LoggerContext loggerContext, ConfigurationSource source) {
-		if (source != null && source != ConfigurationSource.NULL_SOURCE
+		if (source == null || source == ConfigurationSource.NULL_SOURCE) {
-				&& LoggingSystem.get(loggerContext.getClass().getClassLoader()) != null) {
-			return new SpringBootConfiguration();
-		}
 			return null;
 		}
+		return new SpringBootConfiguration();
+	}
 
 	private static final class SpringBootConfiguration extends DefaultConfiguration {