(DOCSP-25293): note behavior when using missing array (#2045)

* note behavior when using missing array

* Copy review

Author: davidhou17

source/reference/operator/aggregation/arrayElemAt.txt

@@ -27,27 +27,27 @@ Definition
    <aggregation-expressions>` that resolves to an array.
 
    The ``<idx>`` expression can be any valid :ref:`expression
-   <aggregation-expressions>` that resolves to an integer:
-
-   - If the ``<idx>`` expression resolves to zero or a positive integer,
-     :expression:`$arrayElemAt` returns the element at the ``idx``
-     position, counting from the start of the array.
-
-   - If the ``<idx>`` expression resolves to a negative integer,
-     :expression:`$arrayElemAt` returns the element at the ``idx``
-     position, counting from the end of the array.
-
-   If ``idx`` exceeds the array bounds, :expression:`$arrayElemAt` does
-   not return a result.
+   <aggregation-expressions>` that resolves to an integer.
 
    For more information on expressions, see
    :ref:`aggregation-expressions`.
 
 Behavior
 --------
 
-For more information on expressions, see
-:ref:`aggregation-expressions`.
+- If the ``<idx>`` expression resolves to zero or a positive integer,
+  :expression:`$arrayElemAt` returns the element at the ``idx``
+  position, counting from the start of the array.
+
+- If the ``<idx>`` expression resolves to a negative integer,
+  :expression:`$arrayElemAt` returns the element at the ``idx``
+  position, counting from the end of the array.
+
+- If ``idx`` exceeds the array bounds, :expression:`$arrayElemAt` does
+  not return a result.
+
+- If the ``<array>`` expression resolves to an undefined array,
+  :expression:`$arrayElemAt` returns ``null``.
 
 .. list-table::
    :header-rows: 1
@@ -65,6 +65,9 @@ For more information on expressions, see
    * - ``{ $arrayElemAt: [ [ 1, 2, 3 ], 15 ] }``
      - 
 
+   * - ``{ $arrayElemAt: [ "$undefinedField", 0 ] }``
+     - ``null``
+
 Example
 -------