loadChunk: zero extent

**Describe the bug**
Seen with #952: it should be allowed to load a zero-size extent.

```
corrupted double-linked list
```

This should produce an empty shared_ptr / zero-sized array (note that for numpy, only the `0:0` selected axes are set to zero-extent in the returned array).

**To Reproduce**

... having a hard time to write a reproducer outside of the location linked above..
https://github.com/ax3l/openPMD-api/tree/topic-zeroExtendRead
https://github.com/openPMD/openPMD-api/blob/f5dc8e25fcc26b3e78857e7cd30b6056a4b9255c/src/binding/python/openpmd_api/DaskArray.py#L39-L46

Python:
```python
import openpmd_api

# ...
data = rc[0:0]
rc.series_flush()
```
or C++
```C++
#include

// ...
```

**Expected behavior**
At the moment, this crashes (at least in Python). Instead, it should return a zero-sized variable/array.

**Software Environment**
- version of openPMD-api: 0.13.2
- installed openPMD-api via: from source
- operating system: Ubuntu 20.04
- machine: local laptop
- name and version of Python implementation: CPython 3.9.2
- version of HDF5: 1.10.6
- version of ADIOS1: N/A
- version of ADIOS2: 2.7.1
- name and version of MPI: N/A

**Additional context**
Seen with #952: This is used by Dask to "probe" the data set on initial access.