Previously, if a user had zero total starred messages,
we would still show the "Unstar all messages" in the
left sidebar on opening the starred messages popover.
This commit adds a check to show button only if the
user had non-zero starred messages. This is done
because-
1. The button, when shown when the user has zero
starred messages, is redundant and may be confusing.
2. Clicking on the button when having zero starred
messages sends a zero-length array to the backend,
resulting in HTTP 400 error.