console.lua: don't error if compute_bounds doesn't return dimensions

compute_bounds doesn't return any field if the rectangle is empty: its
docs say "If set to true, attempt to determine bounds and write them to
the command's result value as x0, x1, y0, y1 rectangle (default:
false). If the rectangle is empty, not known, or somehow degenerate, it
is not set.". So shrinking mpv as much as possible with a menu open
caused a crash.

This doesn't happen on Xorg because of these lines in video/out/x11_common.c:

// Set minimum height/width to 4 to avoid off-by-one errors.
hint->flags |= PMinSize;
hint->min_width = hint->min_height = 4;

nor on macOS because of

minSize = NSSize(width: 160, height: 90)

in video/out/mac/window.swift.

But this is needed on Wayland and Windows.

Fixes #16682 along with the diff posted there.