[2.3] [Contributing] Added note about empty returns

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    |
| Fixed tickets |

contributing/code/standards.rst

@@ -112,6 +112,9 @@ Structure
 * Add a blank line before ``return`` statements, unless the return is alone
   inside a statement-group (like an ``if`` statement);
 
+* Use just ``return;`` instead of ``return null;`` when a function must return void
+  early;
+
 * Use braces to indicate control structure body regardless of the number of
   statements it contains;