tomlkit does not serialize some empty arrays and dictionaries

```python
#!/usr/bin/env python3

import tomlkit


doc = tomlkit.loads("""
x.y.z = 1

[[a]]
x = 1
""")

doc["a"].pop()
doc["x"]["y"].pop("z")

print(f"{doc=}")

rendered = tomlkit.dumps(doc)
print(f"{rendered=}")
```
result
```
doc={'x': {'y': {}}, 'a': []}
rendered='\n\n'
```
Two newlines is not a good TOML representation of that dictionary