Tolerate possible null Flyway MigrationVersion when using Flyway 4.0

Flyway 4.0 provides support for repeatable migrations that do not
have a version. When such a migration has been performed,
MigrationInfo.getMigrationVersion() will return null and, previously,
FlywayEndpoint would fail with an NPE.

This commit updates FlywayEndpoint to use null as the version when
MigrationInfo.getMigrationVersion() returns null.

Closes gh-5700

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/FlywayEndpoint.java

@@ -34,6 +34,7 @@ import org.springframework.util.Assert;
  *
  * @author Eddú Meléndez
  * @author Phillip Webb
+ * @author Andy Wilkinson
  * @since 1.3.0
  */
 @ConfigurationProperties(prefix = "endpoints.flyway")
@@ -80,7 +81,8 @@ public class FlywayEndpoint extends AbstractEndpoint<List<FlywayMigration>> {
 		public FlywayMigration(MigrationInfo info) {
 			this.type = info.getType();
 			this.checksum = info.getChecksum();
-			this.version = info.getVersion().toString();
+			this.version = info.getVersion() != null ? info.getVersion().toString()
+					: null;
 			this.description = info.getDescription();
 			this.script = info.getScript();
 			this.state = info.getState();