HHH-20782 Fix NPE for @FilterJoinTable and @ManyToMany in HQL joins

[rafalsbs]

HHH-20782

Forward-port of the 7.4 fix for HHH-20782.

A NullPointerException is thrown during SQL AST conversion / filter handling when enabled filters are applied to join-table restrictions for explicit HQL joins.

Affects:

• 7.4.5.Final and later

Expected behavior:

• Enabled filters should be applied correctly to join-table restrictions for explicit HQL joins.

Reproducer:

• Standalone JUnit test using H2 with:
  • GroupEntity using @FilterDef and @FilterJoinTable
  • UserEntity with groups
  • GroupEntity with many-to-many masters association

Fails before fix:

• from UserEntity u join u.groups g order by g.id
• from UserEntity u join u.groups g join g.masters m order by m.id

Works:

• from GroupEntity g order by g.id
• from UserEntity u order by u.id

Fix:

• Added the missing null check during filter handling for explicit HQL joins to avoid the NullPointerException during SQL AST conversion.
• Added regression tests covering the failing scenarios.

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Please make sure that the following tasks are completed:
Tasks specific to HHH-20782 (Bug):

• Add test reproducing the bug
• Add entries as relevant to migration-guide.adoc OR check there are no breaking changes

https://hibernate.atlassian.net/browse/HHH-20782