Fix issue #774: Use coverage array for calculation of valid_percent

[MarcelCode]

Summary
This PR updates the calculation of valid_percent in get_array_statistics. Previously, valid_percent was calculated by dividing the number of valid pixels by the total size of the array. Now, it uses the sum of nonzero values in the coverage array for more accurate results.

Why this change?
The previous method incorrectly calculated valid_percent when the coverage array was created using a geometry that wasn’t a simple rectangle. This change ensures the calculation properly reflects the valid area within the intended geometry.

Related Issue
See issue #774 for a detailed explanation of the problem and the motivation for this change.

[vincentsarago]

@MarcelCode thanks for the PR 🙏

Because it's based on the master branch which has some breaking change (dropping python 3.8 support), If I wanted to release this change as is, I would need to publish a 7.3.0 version (minor version update), would this be fine for you?

I can backport this fix in a 7.2.3 if needed

[MarcelCode]

I think a new minor version is fine. No need to backport.
I will pull the new version for our current projects, when it is done.

[vincentsarago]

Thanks @MarcelCode
I'll wait a few days to see if we can get #764 merged before doing a new release

hope that's fine