ACL: AUTH + no default user password raises an error.

This way the behavior is very similar to the past one.
This is useful in order to remember the user she probably failed to
configure a password correctly.

src/server.c

@@ -2901,6 +2901,15 @@ void authCommand(client *c) {
      * will just use "default" as username. */
     robj *username, *password;
     if (c->argc == 2) {
+        /* Mimic the old behavior of giving an error for the two commands
+         * from if no password is configured. */
+        if (DefaultUser->flags & USER_FLAG_NOPASS) {
+            addReplyError(c,"AUTH <password> called without any password "
+                            "configured for the default user. Are you sure "
+                            "your configuration is correct?");
+            return;
+        }
+
         username = createStringObject("default",7);
         password = c->argv[1];
     } else {

tests/unit/auth.tcl

@@ -2,14 +2,14 @@ start_server {tags {"auth"}} {
     test {AUTH fails if there is no password configured server side} {
         catch {r auth foo} err
         set _ $err
-    } {ERR*no password*}
+    } {ERR*any password*}
 }
 
 start_server {tags {"auth"} overrides {requirepass foobar}} {
     test {AUTH fails when a wrong password is given} {
         catch {r auth wrong!} err
         set _ $err
-    } {ERR*invalid password}
+    } {WRONGPASS*}
 
     test {Arbitrary command gives an error when AUTH is required} {
         catch {r set foo bar} err