Fix possible crash with auto-forwarding branches (#4565)

- **PR Description**

Fix crash in AutoForwardBranches when the branches slice is empty.

I'm not aware of any real scenario where this can happen, but we have
seen one stack trace where it crashed with an out-of-bounds error in the
range expression below, so there must be a way. And it seems better to
guard against it anyway, rather than assuming it can't happen.

(Hopefully) fixes #4564.

Author: stefanhaller

pkg/gui/controllers/helpers/branches_helper.go

@@ -270,8 +270,12 @@ func (self *BranchesHelper) AutoForwardBranches() error {
 		return nil
 	}
 
-	allBranches := self.c.UserConfig().Git.AutoForwardBranches == "allBranches"
 	branches := self.c.Model().Branches
+	if len(branches) == 0 {
+		return nil
+	}
+
+	allBranches := self.c.UserConfig().Git.AutoForwardBranches == "allBranches"
 	updateCommands := ""
 	// The first branch is the currently checked out branch; skip it
 	for _, branch := range branches[1:] {