fix: account for global block in is_empty

[paoloricciuti]

Closes #14675

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[dummdidumm]

this is why I voted for the separate walk - it gets hairy to pass this through to everything. Anyway let's go with this.

[paoloricciuti]

this is why I voted for the separate walk - it gets hairy to pass this through to everything. Anyway let's go with this.

i think this should be it because we basically have to account for it when checking for is_used...i also thought of incorporating it in the is_used function but we do have to account for it in the keyframes rule too.

Let's see, if it's get's to complex we can refactor it to use walk.