Allow falsey values for attribute value in a picture-elements card element.

[boralyl]

Proposed change

Allow attribute values that are falsey (0, false, '') for picture elements when using the attribute key. e.g.

cards:
  -   type: picture-elements
      image: "/local/tp-04.png"
      elements:
      - type: state-label
        entity:  air_quality.master_bedroom
        attribute: particulate_matter_2_5
        suffix: ' PM2.5'
        style:
          # ...

In this example the value of 0 is completely valid, but the card will show a warning instead of the value of 0.

Type of change

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

Example configuration

cards:
  -   type: picture-elements
      image: "/local/tp-04.png"
      elements:
      - type: state-label
        entity:  air_quality.master_bedroom
        attribute: particulate_matter_2_5
        suffix: ' PM2.5'
        style:
          # ...

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue or discussion:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• 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