Wrong bonding between residues surrounding missing ones

This is a residue that appears right before several missing residues:

```python
mol = mdt.from_pdb('3p3k')
res = mol.residues[46]
```

MDT currently shows a bond between this and the next available residue, which is clearly wrong:
```python
>>> res.bonded_residues
[,
]
>>> res.distance(res.next_residue)
3.88332344777 ang
```

Also `res.next_residue` returns `''`, but should raise something along the lines of `StopIteration`, or maybe `MissingResidue` or maybe `PrematureTerminus`

I suppose it could also just return a `MissingResidue` object or something

Reproduce the case with mdt.from_pdb('3p3k') and residue 46, then trace how bonded_residues and next_residue handle gaps in the residue sequence. Confirm the intended missing-residue boundary behavior before changing it; done means no bond spans the missing residues and next_residue reports that boundary consistently.

Written by the indexing model from the issue text.