Add nullable to map.merge (#1439)

Co-authored-by: Matea Pesic <80577904+matea16@users.noreply.github.com>

Author: Josipmrden

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

@@ -171,16 +171,18 @@ RETURN map.from_pairs([["b", 3], ["c", "c"]]) AS map;
 ### `merge()`
 
 The procedure merges two maps into one. If the same key occurs twice, the later
-value will overwrite the previous one.
+value will overwrite the previous one. 
+
+If null is provided as an argument, it will resolve to an empty map.
 
 <Callout type="info">
 This function is equivalent to **apoc.map.merge**.
 </Callout>
 
 {<h4 className="custom-header"> Input: </h4>}
 
-- `first: Map` ➡ A map containing key-value pairs that need to be merged with another map.
-- `second: Map` ➡ The second map containing key-value pairs that need to be merged with the key-values from the first map.
+- `first: mgp.Nullable[Map]` ➡ A map containing key-value pairs that need to be merged with another map.
+- `second: mgp.Nullable[Map]` ➡ The second map containing key-value pairs that need to be merged with the key-values from the first map.
 
 {<h4 className="custom-header"> Output: </h4>}