bug#70989: 29.3; Calling isearch-forward when using Unifont throws error "Invalid font name"

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Cc: me@rodrigomorales.site,  70989@debbugs.gnu.org
> Date: Sun, 19 May 2024 21:00:30 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Thanks, but why not replace all of those with a SPC?  It's simpler, I
> > think.
> 
> No reason in particular, I simply thought it would be more elegant to
> replace the other special characters with a different placeholder.

I'd prefer to replace all the characters we cannot use there with a
SPC, for consistency.

Otherwise, I'm okay with the patch; please feel free to install.