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

[Boruch Baum]

On 2021-05-14 09:39, Eli Zaretskii wrote:
> Can you tell what does the below produce on the TTY frame where you
> have this problem?
>
>   M-: (frame-parameter nil 'display) RET

":0.0"

>   M-: (window-frame (window-normalize-window nil t)) RET

#<frame F139 0x5599a30eaf90>

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