Update array-key-exists.xml with deprecation notice (#5063)

Added deprecation notice for using null as key.

Author: cadic

reference/array/functions/array-key-exists.xml

@@ -69,6 +69,12 @@
      </row>
     </thead>
     <tbody>
+     <row>
+      <entry>8.5.0</entry>
+      <entry>
+       Using <type>null</type> in the <parameter>key</parameter> parameter is deprecated, use an empty string instead.
+      </entry>
+     </row>
      <row>
       <entry>8.0.0</entry>
       <entry>