bug#48408: BUGFIX: window-default-font-height: check for nil string

[Boruch Baum]

In package windows.el, function `window-default-font-height' calls
function `face-font', which can return a nil value, but performs a
`string-equal' operation on the output without checking for that
possibiliy.

I came across this in an unusual case, of having an 'emacs -nw' server
an 'emacsclient -nw', and opening a GUI emacsclient to connect to it.
From that point forward until the fix, every *second* attempt to use the
minibuffer on the 'emacsclient -nw' would fail.

The fix I'm using is:

    (if (and (display-multi-font-p (frame-parameter frame 'display))
             (stringp default-font)  ; <<<---!!!
             (not (string-equal (frame-parameter frame 'font) default-font)))

But there could be other considerations to performing things in another way.

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