Use all nan values to represent a NULL QgsRectangle

[strk]

This was suggested by Benoit in
#54646 (comment)

It may make it easier to spot missing isNull() checks by callers.

I'm pretty sure things will fail but chances are that merging various commits currently in other PRs could show why they are needed.

See for instance GH-54976 and GH-54954

I'm marking this as an API break, correct me if I'm wrong

[strk]

I've rebased to current master branch and also made ::setNull() use NaN to represent the null rectangle and ::isNull() only accepting that representation.

[strk]

QGIS server seem to have a problem with nan values in rectangles:
https://github.com/qgis/QGIS/actions/runs/6610295956/job/17959138261#step:13:301

[strk]

The failure is due to QgsMapRendererJob::prepareJobs not checking to see if the visibleExtent is null or not. When it is null, it obviously fail to reproject.

I guess if the visible extent is null that method should just do nothing ?

\cc @elpaso

[strk]

Beside, mSettings.visibleExtent() is called for every iteration but it doesn't need to ?

[strk]

I think the problem is with the semantic of QgsRectangle::isFinite() that would always be false for null rectangles, making it kind of useless

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