azureblob: fix erroneous Rmdir error "directory not empty"

Before this change Rmdir would check the root rather than the
directory specified for being empty and return "directory not empty"
when it shouldn't have done.

backend/azureblob/azureblob.go

@@ -723,7 +723,7 @@ func (f *Fs) Mkdir(dir string) error {
 // isEmpty checks to see if a given directory is empty and returns an error if not
 func (f *Fs) isEmpty(dir string) (err error) {
 	empty := true
-	err = f.list("", true, 1, func(remote string, object *azblob.BlobItem, isDirectory bool) error {
+	err = f.list(dir, true, 1, func(remote string, object *azblob.BlobItem, isDirectory bool) error {
 		empty = false
 		return nil
 	})