unified return null usages
minor #10717 unified return null usages (fabpot)

This PR was merged into the 2.3 branch.

Discussion
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unified return null usages

| Q             | A
| ------------- | ---
| License       | MIT

This PR unifies the way we return `null` from a function or method:

 * always use `return;` instead of `return null;` (the current code base uses both);
 * never use `return;` at the end of a function/method.

Commits
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d1d569b unified return null usages