Reduce started threads in OnClassCondition

Prior to this commit, OnClassCondition started a thread even if the number of
passed autoconfiguration class candidates never exceeded 1. This commit only
starts a thread if there is actually work to split in half.

See gh-22294

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/condition/OnClassCondition.java

@@ -49,7 +49,7 @@ class OnClassCondition extends FilteringSpringBootCondition {
 		// Split the work and perform half in a background thread if more than one
 		// processor is available. Using a single additional thread seems to offer the
 		// best performance. More threads make things worse.
-		if (Runtime.getRuntime().availableProcessors() > 1) {
+		if (autoConfigurationClasses.length > 1 && Runtime.getRuntime().availableProcessors() > 1) {
 			return resolveOutcomesThreaded(autoConfigurationClasses, autoConfigurationMetadata);
 		}
 		else {