strptime() argument 1 must be str, not None

In case a request is done with the following content:
```

```
There is an exception:
```
Traceback (most recent call last):
File "/srv/radicale/env/lib/python3.5/site-packages/radicale/__init__.py", line 332, in __call__
status, headers, answers = self._handle_request(environ)
File "/srv/radicale/env/lib/python3.5/site-packages/radicale/__init__.py", line 505, in _handle_request
environ, base_prefix, path, user)
File "/srv/radicale/env/lib/python3.5/site-packages/radicale/__init__.py", line 982, in do_REPORT
base_prefix, path, xml_content, collection)
File "/srv/radicale/env/lib/python3.5/site-packages/radicale/xmlutils.py", line 1267, in report
multistatus):
File "/srv/radicale/env/lib/python3.5/site-packages/radicale/xmlutils.py", line 1241, in retrieve_items
yield from collection.get_all_filtered(filters)
File "/srv/radicale/env/src/radicale-storage-by-index/radicale_storage_by_index/__init__.py", line 216, in get_all_filtered
datetime.strptime(request['dtend'], "%Y%m%dT%H%M%SZ"))
TypeError: strptime() argument 1 must be str, not None
```

This is caused by line 196 which adds dtend to the request always in case there is a time range in the content.
But in the example request there is only a start and no end..

Easy fix is to ONLY add dtend to request if filter_.get("end") is not None

Start in radicale_storage_by_index/__init__.py at get_all_filtered, especially the request handling around line 196 and the datetime parsing shown in the traceback. Reproduce the calendar-query request with a start but no end, then verify that the request completes without the None strptime error and that bounded ranges still work.

Written by the indexing model from the issue text.