finance/statement: make allowance_to optional

If an excursion has zero admissable youth leaders, the statement and hence the excursion
currently can't be saved, because the `allowance_to` field has to be non-empty, but
the number of entries has to be zero. This commit hence makes the field optional.

jdav_web/finance/migrations/0007_alter_statement_allowance_to.py

@@ -0,0 +1,19 @@
+# Generated by Django 4.0.1 on 2025-01-18 22:00
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('members', '0033_freizeit_approved_extra_youth_leader_count'),
+        ('finance', '0006_statement_add_allowance_to_subsidy_to'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='statement',
+            name='allowance_to',
+            field=models.ManyToManyField(blank=True, help_text='The youth leaders to which an allowance should be paid. The count must match the number of permitted youth leaders.', related_name='receives_allowance_for_statements', to='members.Member', verbose_name='Pay allowance to'),
+        ),
+    ]

jdav_web/finance/models.py

@@ -61,6 +61,7 @@ class Statement(CommonModel):
 
     allowance_to = models.ManyToManyField(Member, verbose_name=_('Pay allowance to'),
                                           related_name='receives_allowance_for_statements',
+                                          blank=True,
                                           help_text=_('The youth leaders to which an allowance should be paid. The count must match the number of permitted youth leaders.'))
     subsidy_to = models.ForeignKey(Member, verbose_name=_('Pay subsidy to'),
                                    null=True,