Improve MuDst accessors to prevent using nullptrs

While discussing the FwdPicoDst integration several places where MuDst uses potentially unsafe pointers came up, see for example:
- primaryTracks()
- primaryVertex()

```
static StMuPrimaryVertex* primaryVertex() { return (StMuPrimaryVertex*)arrays[muPrimaryVertex]->UncheckedAt(mCurrVertexId); }
```

e.g.
`arrays[muPrimaryVertex]` can be null in rare cases