feature #6701 [CS] Avoid using useless expressions (phansys)

This PR was merged into the 2.7 branch.

Discussion
----------

[CS] Avoid using useless expressions

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | N/A

Do not use `else`, `elseif`, `break` after `if` and `case` conditions
which returns or throws something.

Commits
-------

d96faa4 Avoid using useless expressions

Author: wouterj

contributing/code/standards.rst

@@ -175,6 +175,9 @@ Structure
   switched to opt-in via ``@`` operator.
   Read more at :ref:`contributing-code-conventions-deprecations`.
 
+* Do not use ``else``, ``elseif``, ``break`` after ``if`` and ``case`` conditions
+  which return or throw something.
+
 Naming Conventions
 ------------------