Update Fitbit to avoid a KeyError when restingHeartRate is not present

[allenporter]

Proposed change

Update Fitbit to avoid a KeyError when restingHeartRate is not present. I observed this on my own instance where for some days the heart rate data did not include any resting heart rate. Adds a helper function to only get the value when it is present in the response.

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)
• Deprecation (breaking change to happen in the future)
• 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.
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• There is no commented out code in this PR.
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• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

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[jpbede]

LGTM. Thanks 👍