split: prevent panic when store is not set

Prevent panic when split is called but the msgstore is either not
initialized or has no UIDs. This condition could be triggered by calling
:split or :vsplit immediately at startup.

Use the same logic as the :view command, which could operate in a
similar manner.

Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Acked-by: Robin Jarry <robin@jarry.cc>

commands/account/split.go

@@ -30,6 +30,9 @@ func (Split) Execute(aerc *widgets.Aerc, args []string) error {
 	if acct == nil {
 		return errors.New("No account selected")
 	}
+	if acct.Messages().Empty() {
+		return nil
+	}
 	n := 0
 	var err error
 	if len(args) > 1 {