Remove default paths in HAL and Links endpoints as value varies

The default values of fields in @ConfigurationProperties classes are,
where possible, included in the configuration metadata. The default
values for the HAL and Links endpoints vary depending on other
configuration settings. As a result, including a default in the
metadata is misleading.

This commit removes the default assignment of "" to the path fields so
that no default value will be included in the metadata.

Closes gh-3567

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

@@ -38,7 +38,7 @@ public class HalBrowserEndpoint extends WebMvcConfigurerAdapter implements MvcEn
 	private static final String HAL_BROWSER_LOCATION = "classpath:/META-INF/resources/webjars/hal-browser/"
 			+ HAL_BROWSER_VERSION + "/";
 
-	private String path = "";
+	private String path;
 
 	private ManagementServerProperties management;
 

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

@@ -32,7 +32,7 @@ import org.springframework.web.bind.annotation.ResponseBody;
 @ConfigurationProperties("endpoints.links")
 public class LinksMvcEndpoint implements MvcEndpoint {
 
-	private String path = "";
+	private String path;
 	private boolean sensitive = false;
 
 	public LinksMvcEndpoint(String defaultPath) {