Align message with regex for AbstractMvcEndpoint.path

Closes gh-6437

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

@@ -42,7 +42,7 @@ public abstract class AbstractMvcEndpoint extends WebMvcConfigurerAdapter
 	 * Endpoint URL path.
 	 */
 	@NotNull
-	@Pattern(regexp = "/.*|^$", message = "Path must start with /")
+	@Pattern(regexp = "/.*|^$", message = "Path must start with / or be empty")
 	private String path;
 
 	/**