Properly guard DB health indicator

The DataSource health indicator uses `JdbcTemplate` behind the scenes
but nothing was checking that it is actually available.

`DataSourcesHealthIndicatorConfiguration` is now disabled if
`spring-jdbc` is not on the classpath.

Fixes spring-projectsgh-3765

Author: snicoll

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/autoconfigure/HealthIndicatorAutoConfiguration.java

@@ -42,6 +42,7 @@
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
 import org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration;
@@ -57,6 +58,7 @@
 import org.springframework.context.annotation.Configuration;
 import org.springframework.data.mongodb.core.MongoTemplate;
 import org.springframework.data.redis.connection.RedisConnectionFactory;
+import org.springframework.jdbc.core.JdbcTemplate;
 
 /**
  * {@link EnableAutoConfiguration Auto-configuration} for {@link HealthIndicator}s.
@@ -94,6 +96,7 @@ public HealthIndicator applicationHealthIndicator() {
 	}
 
 	@Configuration
+	@ConditionalOnClass(JdbcTemplate.class)
 	@ConditionalOnBean(DataSource.class)
 	@ConditionalOnProperty(prefix = "management.health.db", name = "enabled", matchIfMissing = true)
 	public static class DataSourcesHealthIndicatorConfiguration {