Nyall Dawson 65827adc7d Provide an optimised constructor for a null QgsRectangle
This is used a lot, yet the current constructor calls the
normalize() method which does a bunch of operations for no
result.

So instead provide a simple optimised null QgsRectangle
constructor and save a lot of cycles.

Refs #17809
2018-02-12 08:56:12 +10:00
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