Prevent erroneous "empty locations" log warnings

Update our `ResourceHttpRequestHandler` subclass so that the following
warning is no longer displayed:

  "Locations list is empty. No resources will be served unless a
  custom ResourceResolver is configured as an alternative to
  PathResourceResolver."

Fixes gh-6791

Author: philwebb

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

@@ -106,6 +106,7 @@ private static class Handler extends ResourceHttpRequestHandler {
 
 		Handler(Resource resource) {
 			this.resource = resource;
+			getLocations().add(resource);
 			try {
 				afterPropertiesSet();
 			}
@@ -114,6 +115,11 @@ private static class Handler extends ResourceHttpRequestHandler {
 			}
 		}
 
+		@Override
+		protected void initAllowedLocations() {
+			this.getLocations().clear();
+		}
+
 		@Override
 		protected Resource getResource(HttpServletRequest request) throws IOException {
 			return this.resource;