while: Restore tolerance of condition error

Since commit 880ca66b (Fix: `while()` can silently ignore incorrect
condition, 2021-08-09, v3.22.0-rc1~238^2~4) we correctly reject the
code

    set(paren "(")
    while(${paren})
    endwhile()

However, rejecting it breaks compatibility with projects that used such
code accidentally.  In CMake 3.21 and below, any error in the condition
was ignored because the `false` result exited the loop first.  Restore
tolerance of the error for now.  A policy will be needed to make it an
error later.

Note that the same condition with `if` was always correctly rejected.

Fixes: #22524
Issue: #23296
Co-authored-by: Brad King <brad.king@kitware.com>
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