[READY] Handle null characters in completion response
The `detailed_info` field may contain null characters e.g. `\x00` in Python docstrings (though it doesn't make sense to use such characters in a docstring). Since these characters cannot be evaluated through `pyeval` (and `py3eval`), we replace them with the symbol used by Vim to display them (`^@`).
This PR also rewrites the `_ConvertCompletionDataToVimData` function by relying on two things:
- [the `insertion_text` field is always defined](600f54de20/ycmd/responses.py (L109));
- [Vim ignores fields with an empty string](833e5dab14/src/edit.c (L2748-L2755)).
I did some measurements and the performance hit is negligible.
Fixes#3207.
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