Nil pointer panic in GetRemoteBranchListDisplayStrings

Hi,

I've been looking at the crash reported in #5240 and traced it to `pkg/gui/presentation/remote_branches.go`. The `GetRemoteBranchListDisplayStrings` function doesn't guard against nil elements in the `branches` slice before calling `branch.FullName()`.

This can happen during a race between remote editing/refreshing — when the remote list is updated concurrently, stale branch pointers can end up as nil in the slice.

A simple nil check before accessing the branch would prevent the panic.