docs: Align collections NULL-argument handling with compatibility layer

pages/advanced-algorithms/available-algorithms/collections.mdx

@@ -39,6 +39,44 @@ uncomparable. <br />
 Only `Numeric` data types can be used in `sum()` and `avg()` functions, so only
 `Int` and `Double` data types are allowed.
 
+### Handling `NULL` values
+
+Passing `NULL` as a whole list (or scalar) argument no longer raises an
+argument-validation error. Each function returns a defined result:
+
+| Function | Result when a list argument is `NULL` |
+| --- | --- |
+| `sum()`, `sum_longs()`, `avg()`, `min()`, `max()`, `to_set()`, `pairs()` | `null` |
+| `union()`, `union_all()` | the other list; `null` when both are `NULL` |
+| `remove_all()` | `null` when the first list is `NULL`; the first list when the second is `NULL` |
+| `intersection()`, `sort()`, `flatten()` | empty list `[]` |
+| `contains()`, `contains_all()`, `contains_sorted()` | `false` |
+| `frequencies_as_map()` | empty map `{}` |
+| `split()`, `partition()` | no rows |
+
+`NULL` elements inside a list are handled per function:
+
+- `min()` and `max()` skip `NULL` elements; a list containing only `NULL` values returns `null`.
+- `contains()` and `contains_all()` never match a `NULL` search value (`NULL = NULL` is not true), so searching for `NULL` returns `false`.
+- `split()` treats a `NULL` delimiter as matching nothing and returns the whole list as a single part; `NULL` list elements are kept.
+- `frequencies_as_map()` counts `NULL` elements under the `"NO_VALUE"` key.
+- `flatten()`, `pairs()`, `to_set()`, `union()`, `union_all()`, `remove_all()` and `intersection()` keep `NULL` elements as ordinary values.
+- `sum()`, `sum_longs()`, `avg()`, `sort()` and `contains_sorted()` still reject a list that contains a `NULL` element; `contains_sorted()` also rejects a `NULL` search value, and `partition()` still rejects a `NULL` size.
+
+For example, a `NULL` argument returns the defined result instead of erroring:
+
+```cypher
+RETURN collections.sum(null) AS sum, collections.sort(null) AS sorted;
+```
+
+```plaintext
++----------------------------+
+| sum        | sorted        |
++----------------------------+
+| null       | []            |
++----------------------------+
+```
+
 ### `sort()`
 
 Sorts the elements of an input list if they are of the same data type. For the input