__fish_cursor_xterm: Ignore unknown cursor settings

This prevents leaking the escape sequence by printing nonsense, and it
also allows disabling cursor setting by just setting the variable to
e.g. empty.

And if we ever added any shapes, it would allow them to be used on new
fish and ignored on old

Fixes #9698

share/functions/__fish_cursor_xterm.fish

@@ -8,6 +8,9 @@ function __fish_cursor_xterm -d 'Set cursor (xterm)'
             set shape 4
         case line
             set shape 6
+        case '*'
+            # Unknown shape
+            return
     end
     if contains blink $argv
         set shape (math $shape - 1)