[BUG] opm: opm diff does not prune empty packages in headsonly mode.

When running `opm alpha diff` without a previous reference, if an empty package is produced due to an input error (e.g., version does not exist in any channels), it should be pruned to ensure the file-based catalog is valid.

Example of a produced catalog (icon removed):
```
{
  "schema": "olm.package",
  "name": "cluster-logging",
  "defaultChannel": "stable",
}
