Fix mongo database error

[st01cs]

Description

response = await self.database[self.collection].find_one()

[response] will be None when cant find document in collection, response.key() will throw exception.

Status

READY

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Find an document which does not exist in collection, Opsdroid will not be broken.

Checklist:

• [ *] I have performed a self-review of my own code
• [ *] I have made corresponding changes to the documentation (if applicable)
• [ *] I have added tests that prove my fix is effective or that my feature works
• [ *] New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 99.14%. Comparing base (e2ffb0c) to head (ff31730).
Report is 6 commits behind head on master.

❗ Current head ff31730 differs from pull request most recent head 3c375ba. Consider uploading reports for the commit 3c375ba to get more accurate results

Files	Patch %	Lines
opsdroid/database/mongo/__init__.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2015      +/-   ##
==========================================
- Coverage   99.16%   99.14%   -0.02%     
==========================================
  Files          83       83              
  Lines        5395     5397       +2     
==========================================
+ Hits         5350     5351       +1     
- Misses         45       46       +1     

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[jacobtomlinson]

Sorry for the delay in reviewing here. Thanks!