BUGFIX: Guard that Fusion path cannot be empty

[mhsdesign]

Previously in php 7.4 this Neos\Fusion\Exception\MissingFusionObjectException was thrown

No Fusion object found in path ""

but with php 8 this ValueError is thrown which is unexpected

strrpos(): Argument #3 ($offset) must be contained in argument #1 ($haystack)

This change takes care of throwing an explicit Neos\Fusion\Exception instead:

Fusion path cannot be empty.

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This error was noticed in the out of band rendering, when there is no content element wrapping: https://discuss.neos.io/t/argument-3-offset-must-be-contained-in-argument-1-haystack/6416/4

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Checklist

• Code follows the PSR-2 coding style
• Tests have been created, run and adjusted as needed
• The PR is created against the lowest maintained branch
• Reviewer - PR Title is brief but complete and starts with FEATURE|TASK|BUGFIX
• Reviewer - The first section explains the change briefly for change-logs
• Reviewer - Breaking Changes are marked with !!! and have upgrade-instructions