`sync` fails when a database has no sequences.

**Describe the bug**
If a source DB does not have any sequences, when running `sync` it bombs out.

**Excerpt**

```
AttributeError: 'NoneType' object has no attribute 'decode'
```

Seems `pgbelt.utils.postgres.py` `load_sequences` always tries to run even with an empty dict. Need to not fail on that.