Reflect `IS NULL`/`IS NOT NULL` usage for derived queries using `null` as argument.

Closes #3674

src/main/antora/modules/ROOT/pages/jpa/query-methods.adoc

@@ -44,7 +44,7 @@ The following table describes the keywords supported for JPA and what a method c
 |`Distinct`|`findDistinctByLastnameAndFirstname`|`select distinct ... where x.lastname = ?1 and x.firstname = ?2`
 |`And`|`findByLastnameAndFirstname`|`… where x.lastname = ?1 and x.firstname = ?2`
 |`Or`|`findByLastnameOrFirstname`|`… where x.lastname = ?1 or x.firstname = ?2`
-|`Is`, `Equals`|`findByFirstname`,`findByFirstnameIs`,`findByFirstnameEquals`|`… where x.firstname = ?1`
+|`Is`, `Equals`|`findByFirstname`,`findByFirstnameIs`,`findByFirstnameEquals`|`… where x.firstname = ?1` (or `… where x.firstname IS NULL` if the argument is `null`)
 |`Between`|`findByStartDateBetween`|`… where x.startDate between ?1 and ?2`
 |`LessThan`|`findByAgeLessThan`|`… where x.age < ?1`
 |`LessThanEqual`|`findByAgeLessThanEqual`|`… where x.age \<= ?1`