find_one() modified to return None instead of {} when nothing found

[IlyaSkriblovsky]

This change relates only to find_one, find still returns empty list when nothing was found

[coveralls]

Coverage decreased (-0.22%) to 90.94% when pulling 29c701b on IlyaSkriblovsky:findone-none into 137b363 on twisted:master.

[coveralls]

Coverage decreased (-0.22%) to 90.94% when pulling 29c701b on IlyaSkriblovsky:findone-none into 137b363 on twisted:master.

[IlyaSkriblovsky]

Looks like we have some lines which have unstable coverage. Sometimes they are visited during some tests and sometimes not. I mean these: https://coveralls.io/builds/2651523/source?filename=txmongo%2Fprotocol.py#L322
They were occasionally visited during previous build but not visited now, so coverage is decreased a bit.

[psi29a]

I've seen those as well, likely the way those versions for Twisted work.