Hibernate 7: support DB2 in the unique key null handling, and test it #16746

Problem

The code that chooses how a unique key is built or read for a nullable key treats DB2 like H2, PostgreSQL, MySQL, MariaDB and SQL Server: a row with a null in the key never conflicts. That is not verified for DB2 and may be wrong.

Hibernate creates DB2's unique key as create unique index ... exclude null keys (DB2Dialect#getCreateIndexTail; AlterTableUniqueIndexDelegate says DB2 does not allow unique constraints on nullable columns). As far as I know, exclude null keys leaves out an entry only when all of its key columns are null, and DB2 compares the other nulls as equal, so ('en', null) twice would conflict, as on Oracle. Nobody has run this on DB2.

Where the assumption is made

• UniqueIndexNulls (Hibernate 7: write the rows of unidirectional hasMany collections that share a join table #16701): maps only OracleDialect to COMPARED, every other dialect, DB2 included, to DISTINCT. SharedJoinTableUniqueness builds one unique key per association for DISTINCT. On DB2, if nulls are compared, two associations that share a key column, or owners that share an element, would be rejected on legitimate rows.
• UniqueNullsPerDialect for Hibernate 5 and 7 (Validate a unique group with a null value against the rows whose group value is null #16583, issue Unique Constraint with multiple columns ignores null value and throws validation error #14503): DB2 reads skip. If DB2 compares a partly null key, database should read value there, and the DB2 row of the table in unique.adoc and in the Grails 9 upgrade note is wrong.

What is needed

1. A DB2 container spec (Testcontainers db2 module, icr.io/db2_community/db2) added to the database matrix of SharedJoinTableHibernate7Spec, SharedKeyColumnHibernate7Spec and UniqueNullsHibernate7Spec, so the behavior is measured and not read from the source.
2. If DB2 compares partly null keys, give it the COMPARED / value reading, with the DDL in SharedJoinTableUniqueKeyBinderSpec adjusted, and correct the documentation tables.
3. Say in the documentation which databases the tests cover.

The unit-level DDL specs only show the DDL Hibernate renders. They do not show how DB2 treats the index.