docs: add changelog for PR 13640

changes/ce/feat-13640.en.md

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+Add two new rule SQL functions `coalesce/2` and `coalesce_ne/2`.
+
+The two functions can help to simplify null-checks in rule SQL.
+For example
+
+```
+SELECT
+    CASE
+        WHEN is_null(payload.path.to.value) THEN
+            0
+        ELSE
+            payload.path.to.value
+    END AS my_value
+```
+
+can be as simple as: `SELECT coalesce(payload.path.to.value, 0) AS my_value`.
+