Reinstate support for Hibernate < 5.5

See gh-27352

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/orm/jpa/HibernateProperties.java

@@ -173,18 +173,18 @@ public class HibernateProperties {
 
 		private void applyNamingStrategies(Map<String, Object> properties) {
 			applyNamingStrategy(properties, AvailableSettings.IMPLICIT_NAMING_STRATEGY, this.implicitStrategy,
-					SpringImplicitNamingStrategy.class.getName());
+					() -> SpringImplicitNamingStrategy.class.getName());
 			applyNamingStrategy(properties, AvailableSettings.PHYSICAL_NAMING_STRATEGY, this.physicalStrategy,
-					CamelCaseToUnderscoresNamingStrategy.class.getName());
+					() -> CamelCaseToUnderscoresNamingStrategy.class.getName());
 		}
 
 		private void applyNamingStrategy(Map<String, Object> properties, String key, Object strategy,
-				Object defaultStrategy) {
+				Supplier<String> defaultStrategy) {
 			if (strategy != null) {
 				properties.put(key, strategy);
 			}
-			else if (defaultStrategy != null && !properties.containsKey(key)) {
-				properties.put(key, defaultStrategy);
+			else {
+				properties.computeIfAbsent(key, (k) -> defaultStrategy.get());
 			}
 		}
 

spring-boot-tests/spring-boot-smoke-tests/spring-boot-smoke-test-hibernate52/src/main/resources/application.properties

@@ -0,0 +1 @@
+spring.jpa.hibernate.naming.physical-strategy=org.hibernate.boot.model.naming.PhysicalNamingStrategyStandardImpl