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`
@@ -60,7 +60,7 @@ The following table describes the keywords supported for JPA and what a method c
 |`EndingWith`|`findByFirstnameEndingWith`|`… where x.firstname like ?1` (parameter bound with prepended `%`)
 |`Containing`|`findByFirstnameContaining`|`… where x.firstname like ?1` (parameter bound wrapped in `%`)
 |`OrderBy`|`findByAgeOrderByLastnameDesc`|`… where x.age = ?1 order by x.lastname desc`
-|`Not`|`findByLastnameNot`|`… where x.lastname <> ?1`
+|`Not`|`findByLastnameNot`|`… where x.lastname <> ?1` (or `… where x.firstname IS NOT NULL` if the argument is `null`)
 |`In`|`findByAgeIn(Collection<Age> ages)`|`… where x.age in ?1`
 |`NotIn`|`findByAgeNotIn(Collection<Age> ages)`|`… where x.age not in ?1`
 |`True`|`findByActiveTrue()`|`… where x.active = true`