doc/contributing: add lib.optional (#121251)

Co-authored-by: Cole Helbling <cole.e.helbling@outlook.com>

doc/contributing/coding-conventions.chapter.md

@@ -181,6 +181,21 @@
   rev = "${version}";
   ```
 
+- Filling lists condionally _should_ be done with `lib.optional(s)` instead of using `if cond then [ ... ] else null` or `if cond then [ ... ] else [ ]`.
+
+  ```nix
+  buildInputs = lib.optional stdenv.isDarwin iconv;
+  ```
+
+  instead of
+
+  ```nix
+  buildInputs = if stdenv.isDarwin then [ iconv ] else null;
+  ```
+
+  As an exception, an explicit conditional expression with null can be used when fixing a important bug without triggering a mass rebuild.
+  If this is done a follow up pull request _should_ be created to change the code to `lib.optional(s)`.
+
 - Arguments should be listed in the order they are used, with the exception of `lib`, which always goes first.
 
 ## Package naming {#sec-package-naming}