Make isPresent check more Graal-friendly

Closes gh-23925

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/liquibase/LiquibaseServiceLocatorApplicationListener.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -37,10 +37,13 @@ public class LiquibaseServiceLocatorApplicationListener implements ApplicationLi
 
 	private static final Log logger = LogFactory.getLog(LiquibaseServiceLocatorApplicationListener.class);
 
+	private static final boolean LIQUIBASE_PRESENT = ClassUtils.isPresent(
+			"liquibase.servicelocator.CustomResolverServiceLocator",
+			LiquibaseServiceLocatorApplicationListener.class.getClassLoader());
+
 	@Override
 	public void onApplicationEvent(ApplicationStartingEvent event) {
-		if (ClassUtils.isPresent("liquibase.servicelocator.CustomResolverServiceLocator",
-				event.getSpringApplication().getClassLoader())) {
+		if (LIQUIBASE_PRESENT) {
 			new LiquibasePresent().replaceServiceLocator();
 		}
 	}