Do not use the lower revision when selected revision is empty

Closes #12472
(grafted from 17ff80636eb21be4fd90d1de98ce05e0c274be87)

--HG--
branch : 6.0

sao/src/tab.js

@@ -1005,6 +1005,7 @@
                         revision.add(1, 'milliseconds');
                     }
                     if ((this.screen.current_view.view_type == 'form') &&
+                            revision &&
                             (revision < revisions[revisions.length - 1][0])) {
                         revision = revisions[revisions.length - 1][0];
                     }