Handle None values in weather entity forecast

[rianadon]

Proposed change

I've realized that some of the weather components pass in None values to forecast attributes. This currently causes the unit conversions to fail if the units are configured, since the unit conversion methods cannot handle None.

To be clear, nothing is currently failing because of this issue. However, I am running into this problem when fixing my PRs to update the weather unit conversion in components.

I figure it's easier to handle None values inside the weather entity rather than in every component.

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

Additional information

• This PR fixes or closes issue: fixes #
• 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.
• For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.
• Untested files have been added to .coveragerc.

The integration reached or maintains the following Integration Quality Scale:

• No score or internal
• 🥈 Silver
• 🥇 Gold
• 🏆 Platinum

To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[probot-home-assistant]

Hey there @fabaff, mind taking a look at this pull request as it has been labeled with an integration (weather) you are listed as a code owner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[frenck]

LGTM, this also matches the developer docs.