Render commitMessage in title attribute only if it exists in JSON

[`gerrit query` command documentation](https://review.openstack.org/Documentation/cmd-query.html) page states:

> Note that any field may be missing in the JSON messages, so consumers of this
> JSON stream should deal with that appropriately.

so we additionally check if `commitMessage` field is present in JSON response.

Change-Id: I970326b8cb41294fe6dedeeb88e1e4f249575c64
Signed-off-by: Jindrich Besta <jindrich@amarulasolutions.com>

src/main/resources/com/sonyericsson/hudson/plugins/gerrit/trigger/hudsontrigger/actions/manual/ManualTriggerAction/index.jelly

@@ -150,7 +150,7 @@
                                                             ${patch.getString('number')}
                                                         </td>
                                                         <td onClick="activateRow('${theId}')">
-                                                            <span title="${res.get('commitMessage')}">
+                                                            <span title="${res.has('commitMessage') ? res.get('commitMessage') : null}">
                                                                 ${it.toReadableHtml(res.getString('subject'))}
                                                             </span>
                                                             <j:if test="${res.has('status')}">