core/bootstrap: fix panic without backup bootstrap peer functions (#10029)

Fix panic when backup bootstrap peer load and save funcs are nil

A panic occurs when the first bootstrap round runs is these functions are not assigned in the configuration:
- `LoadBackupBootstrapPeers`
- `SaveBackupBootstrapPeers`

This fix assumes that it is acceptable for these functions to be nil, as it may be desirable to disable the backup peer load and save functionality.