Update _usageNotes.md

Author: zspitzer

docs/03.reference/02.tags/queryparam/_usageNotes.md

@@ -1 +1,8 @@
-Prior to Lucee 6, empty values would be auto cast to nulls, Since Lucee 6 only strings behave this way [LDEV-4410](https://luceeserver.atlassian.net/browse/LDEV-4410)
+Prior to Lucee 6, empty values would be auto cast to nulls, since Lucee 6 only strings behave this way [LDEV-4410](https://luceeserver.atlassian.net/browse/LDEV-4410)
+
+In Lucee 6, this is no longer the default behavior and throws an exception, which matches ACF.
+
+You can re-enable the old behavior by setting this environment variable or system property to `true`.
+
+**Environment Variable:** `LUCEE_QUERY_ALLOWEMPTYASNULL=TRUE`  
+**System Property:** `-Dlucee.query.allowemptyasnull="true"`