Fix coronavirus worldwide sum

[MartinHjelmare]

Breaking change

Proposed change

• Fix sum for worldwide sensors when an addend is None.
• Return None if any of the addends (countries) are None.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example entry for configuration.yaml:

# Example configuration.yaml

Additional information

• This PR fixes or closes issue: fixes Coronavirus Integration - Worldwide Current and Recovered sensors are unavailable #36716
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

The integration reached or maintains the following Integration Quality Scale:

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[probot-home-assistant]

Hey there @home_assistant/core, mind taking a look at this pull request as its been labeled with a integration (coronavirus) you are listed as a codeowner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[MartinHjelmare]

I'll add some sensor tests.

[stickpin]

@MartinHjelmare, @frenck any planes to merge the fix?

Thanks in advance!

[stickpin]

@frenck can you approve please and include it into the next Milestone? :)

[MartinHjelmare]

I haven't pushed the tests yet. Please be patient and don't spam this PR.