slapos_subscription_request: do not allow empty quantity on Subscription Request

master/bt5/slapos_subscription_request/PropertySheetTemplateItem/portal_property_sheets/SlapOSSubscriptionRequestConstraint/empty_quantity_constraint.xml

+<?xml version="1.0"?>
+<ZopeData>
+  <record id="1" aka="AAAAAAAAAAE=">
+    <pickle>
+      <global name="TALES Constraint" module="erp5.portal_type"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>_identity_criterion</string> </key>
+            <value>
+              <persistent> <string encoding="base64">AAAAAAAAAAI=</string> </persistent>
+            </value>
+        </item>
+        <item>
+            <key> <string>_range_criterion</string> </key>
+            <value>
+              <persistent> <string encoding="base64">AAAAAAAAAAM=</string> </persistent>
+            </value>
+        </item>
+        <item>
+            <key> <string>description</string> </key>
+            <value>
+              <none/>
+            </value>
+        </item>
+        <item>
+            <key> <string>expression</string> </key>
+            <value> <string>python: context.getQuantity(0) == 1</string> </value>
+        </item>
+        <item>
+            <key> <string>id</string> </key>
+            <value> <string>empty_quantity_constraint</string> </value>
+        </item>
+        <item>
+            <key> <string>message_expression_false</string> </key>
+            <value> <string>Quantity must not be empty</string> </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
+  <record id="2" aka="AAAAAAAAAAI=">
+    <pickle>
+      <global name="PersistentMapping" module="Persistence.mapping"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>data</string> </key>
+            <value>
+              <dictionary/>
+            </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
+  <record id="3" aka="AAAAAAAAAAM=">
+    <pickle>
+      <global name="PersistentMapping" module="Persistence.mapping"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>data</string> </key>
+            <value>
+              <dictionary/>
+            </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
+</ZopeData>