Fix extent computation in QgsSpatiaLiteProvider

[strk]

Trying to debug #54646 (comment) I was lost in this function and thought I'd rewrite it.

[lnicola]

Looks like I had an unsubmitted comment for some reason. Looks fine, I guess, but I really think moving to the standard query API is a better idea. But even without that, this looks like an improvement and probably fixes the mentioned issue.

[strk]

The basic fix here, I believe, is handling the NULL min/max values of extent when number of rows is zero.
In that case, without this change, you get a 0.0 values instead of NULL, so a 0,0,0,0 QgsRectangle instead of a Null rectangle.
Nobody notices until you stop considering 0,0,0,0 not-null ( as in #54646 )

[strk]

If the empty string check is not good (CI will tell, I guess?) we can consider using proper null checking. This may be useful in that case: https://stackoverflow.com/questions/8961457/how-to-check-a-value-in-a-sqlite-column-is-null-or-not-with-c-api

[strk]

@elpaso I've addressed all your suggestions, are we ready to go now ? I need one approval to proceed