Invert suggestion if pointer is tested for non-nullness

[samueltardieu]

Fixes #15010

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changelog: [cmp_null]: emit proper suggestion when pointer is checked for non-nullness

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[llogiq]

Thank you!

[samueltardieu]

We should have beta-nominated this. @flip1995 if you hear about a point release for Rust 1.88.1, we should add this one as this fixes a I-suggestion-causes-bug on a warn by default lint, this may produce code with an inverted logic.