subsys/fs: correct misinterpretation of Zephyr API

The Zephyr fs readdir abstraction returns OK with a zero-length file
name when the last directory entry has been found.  The loop to build
multiple entries instead checked for a non-file entry type.

Correct the loop exit condition.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>

subsys/fs/fs.c

@@ -261,6 +261,9 @@ int fs_readdir(struct fs_dir_t *zdp, struct fs_dirent *entry)
 				if (rc < 0) {
 					break;
 				}
+				if (entry->name[0] == 0) {
+					break;
+				}
 				if (entry->type != FS_DIR_ENTRY_DIR) {
 					break;
 				}