slapos_accounting: Accept no ledger (1 or 0) on constraint

    Some documents (like manual accounting) may contain no ledger, so accept that the value is not set.

   Constraint was kept to prevent 2 values been set, or some unexpected set (non-category).

master/bt5/slapos_accounting/PropertySheetTemplateItem/portal_property_sheets/SlapOSLedgerConstraint/ledger_arity_constraint_constraint.xml

@@ -47,7 +47,7 @@ Let\'s try first like this to detect all issues in tests</string> </value>
         </item>
         <item>
             <key> <string>min_arity</string> </key>
-            <value> <int>1</int> </value>
+            <value> <int>0</int> </value>
         </item>
         <item>
             <key> <string>portal_type</string> </key>