Document "nullif" function (#6451)

Author: philrz

book/src/SUMMARY.md

@@ -123,6 +123,7 @@
             - [has](super-sql/functions/generics/has.md)
             - [len](super-sql/functions/generics/len.md)
             - [map](super-sql/functions/generics/map.md)
+            - [nullif](super-sql/functions/generics/nullif.md)
             - [under](super-sql/functions/generics/under.md)
         - [Errors](super-sql/functions/errors/intro.md)
             - [error](super-sql/functions/errors/error.md)

book/src/super-sql/functions/generics/nullif.md

@@ -0,0 +1,38 @@
+### Function
+
+  **nullif** — returns a null value if values are equal
+
+### Synopsis
+
+```
+nullif(val1: any, val2: any) -> any
+```
+
+### Description
+
+The `nullif` function returns a `null` value if its first argument `val1` is
+equal to its second argument `val2`, otherwise it returns `val1`.
+
+### Examples
+
+---
+
+```mdtest-spq
+# spq
+nullif(1, 1)
+# input
+
+# expected output
+null::int64
+```
+
+---
+
+```mdtest-spq
+# spq
+nullif("foo", "bar")
+# input
+
+# expected output
+"foo"
+```