bug#50716: 28.0.50; whitespace-report does not use existing windows

[Gabriel]

Description:
The command 'whitespace-report' does not using existing windows, so
every new invocation opens a new window at bottom. This behavior is
different from how other emacs commands handle windows. I expect
'whitespace-report' to behave similarly to other commands (e.g.:
'describe-variable', 'memory-report', etc). The problem seems to be on
'whitespace-display-window'.

Steps to reproduce:
1) 'emacs -Q' from master branch
2) M-x 'whitespace-report'
3) 'C-x o' to move to "*Whitespace Report*" buffer
4) 'C-x k RET' to kill the "*Whitespace Report*" buffer
5) Repeat steps 2-4 multiple times

--
        Gabriel