feat: allow invalid source data by making gatsbyImageData nullable

[raae]

When sourcing data every sourced item might not be be valid. Instead of stopping the build an error will be logged.

Make sure to check if gatsbyImageData is not null before using in the GatsbyImage component.

Closes #214

[github-actions[bot]]

:tada: This PR is included in version 3.1.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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[github-actions[bot]]

:tada: This PR is included in version 4.2.0 :tada:

The release is available on:

• npm package (@next dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: