Dashboard Tabs: fix action "Move Right"

Summary:
It seems that if you use the "Move Right" action, you can get
an unrelated error message about "Move Left":

This is already the first tab. It can not move any farther to the left.

This happened because there was an incomplete condition.

Closes T15493

Test Plan:

• visit /dashboard/panel/ and create at least two panels (please, owned by you)
• visit a Dashboard, and add a Tabs Panel
• take the first Tab: "Move Right"

After this change, "Move Right" now always works.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15493

Differential Revision: https://we.phorge.it/D25307