Empty comments get dropped by the serializer

A simple fragment like

```properties
## This is a group

g1 = one
g2 = two

##

not = in that group
```

doesn't serialize right. The reason is the side note in https://docs.python.org/3/library/stdtypes.html#str.splitlines, which mentions that `splitlines` returns an empty list for an empty string instead of `[""]`. Pfff.