Problem:    Making comparison with null work changes legacy behavior.
Solution:   Only use the better comparison in Vim9 script. (closes #9910)

Problem:    Vim9: comparing partial with function fails.
Solution:   Support this comparison.  Avoid a crash. (closes #9909)
            Add more test cases.

Problem:    Crash when using null_function for a partial.
Solution:   Don't call fname_trans_sid() with NULL. (closes #9908)

Problem:    The Athena GUI is old and does not work well.
Solution:   Remove the Athena GUI from configure to find out who still wants
            support for this GUI.

Problem:    Vim9: cannot set variables to a null value.
Solution:   Add null_list, null_job, etc.

Problem:    Some GUI tests don't work on Athena.
Solution:   Skip tests that won't work. (Yegappan Lakshmanan, closes #9902)

Problem:    MS-Windows: cannot build with some sodium libraries.
Solution:   Make the DLL name configuragle.  Add build instructions.
            (Ken Takata, closes #9905)

Problem:    When gvim is started maximized the 'window' option isn't set
            properly. (Christian J. Robinson)
Solution:   Check if 'windows' was already set or not. (Ken Takata,
            closes #9904)

Problem:    GUI test fails with Motif. (Dominique Pellé)
Solution:   Remove using an invalid value for 'guifontset'.