Given a tracker with a checkbox field bound to users that is not required and given I created an artifact with nothing checked (no value selected)
when I export the project (using export_project_xml.php) containing this tracker to an archive zip file
then a PARSE ERROR will be thrown.
 
Additional technical info: I created this tracker from scratch and it was a new artifact (only one changeset).
See the error below :
[root@372162dd299a utils]# ./php-launcher.sh export_project_xml.php -p 109 -u admin -t 218 -o /tmp/archive.zip
*** PARSE ERROR: 1: <?xml version="1.0"?>
2: <artifacts>
3:   <artifact id="539">
4:     <changeset>
5:       <submitted_by format="username">jmasson</submitted_by>
6:       <submitted_on format="ISO8601">2015-10-01T15:55:26+02:00</submitted_on>
7:       <comments/>
8:       <field_change field_name="title_1" type="string">
9:         <value>default values</value>
10:       </field_change>
11:       <field_change field_name="sb_users" type="list" bind="users">
12:         <value format="username">mmeshanko</value>
13:       </field_change>
14:       <field_change field_name="msb_users" type="list" bind="users">
15:         <value format="username">jmasson</value>
16:         <value format="username">mmeshanko</value>
17:       </field_change>
18:       <field_change field_name="rb_users" type="list" bind="users">
19:         <value format="username">jmasson</value>
20:       </field_change>
21:       <field_change field_name="cb_users" type="list" bind="users">
22:         <value format="username">mmeshanko</value>
23:       </field_change>
24:       <field_change field_name="ol_users" type="open_list" bind="users">
25:         <value/>
26:       </field_change>
27:     </changeset>
28:   </artifact>
29:   <artifact id="540">
30:     <changeset>
31:       <submitted_by format="username">jmasson</submitted_by>
32:       <submitted_on format="ISO8601">2015-10-01T15:55:47+02:00</submitted_on>
33:       <comments/>
34:       <field_change field_name="title_1" type="string">
35:         <value>Changed values</value>
36:       </field_change>
37:       <field_change field_name="sb_users" type="list" bind="users">
38:         <value format="username">jmasson</value>
39:       </field_change>
40:       <field_change field_name="msb_users" type="list" bind="users">
41:         <value format="username">mmeshanko</value>
42:       </field_change>
43:       <field_change field_name="rb_users" type="list" bind="users">
44:         <value format="username">mmeshanko</value>
45:       </field_change>
46:       <field_change field_name="cb_users" type="list" bind="users">
47:         <value format="username">jmasson</value>
48:         <value format="username">mmeshanko</value>
49:       </field_change>
50:       <field_change field_name="ol_users" type="open_list" bind="users">
51:         <value format="username">jmasson</value>
52:       </field_change>
53:     </changeset>
54:   </artifact>
55:   <artifact id="541">
56:     <changeset>
57:       <submitted_by format="username">jmasson</submitted_by>
58:       <submitted_on format="ISO8601">2015-10-01T15:56:03+02:00</submitted_on>
59:       <comments/>
60:       <field_change field_name="title_1" type="string">
61:         <value>No values</value>
62:       </field_change>
63:       <field_change field_name="sb_users" type="list" bind="users">
64:         <value format="username">None</value>
65:       </field_change>
66:       <field_change field_name="msb_users" type="list" bind="users">
67:         <value format="username">None</value>
68:       </field_change>
69:       <field_change field_name="rb_users" type="list" bind="users">
70:         <value format="username">None</value>
71:       </field_change>
72:       <field_change field_name="cb_users" type="list" bind="users">
73:         <value/>
74:       </field_change>
75:       <field_change field_name="ol_users" type="open_list" bind="users">
76:         <value/>
77:       </field_change>
78:     </changeset>
79:   </artifact>
80: </artifacts>
*** PARSE ERROR: Line: 73 Column: 17  error  required attributes missing
RNG path: /usr/share/tuleap/plugins/tracker/www/resources/artifacts.rng