Fix video and model viewer

[nicolas-daures]

Description

The snapshot of a video with poster that was never played was empty. Now, if the currentTime of the video is 0 and there is a poster, the snapshot is based on the poster.

When a slot tag does not have assignedNodes, the snapshot does not take child nodes into account. The model-viewer (https://modelviewer.dev/) which displays 3D models could therefore not be displayed in a snapshot. Now, if the slot tag does not have assignedNodes, the snapshot uses the childNodes.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Enhancement (changes that improvement of current feature or performance)
• Refactoring (changes that neither fixes a bug nor adds a feature)
• Test Case (changes that add missing tests or correct existing tests)
• Code style optimization (changes that do not affect the meaning of the code)
• Docs (changes that only update documentation)
• Chore (changes that don't modify src or test files)

Self Check before Merge

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.