If a variable existed in `this` but not in `other`, its resulting type would be set to undefined. Changing it to use the type from `this` seems to fix https://github.com/TurboWarp/scratch-vm/issues/273.
If a variable existed in `this` but not in `other`, its resulting type would be set to undefined. Changing it to use the type from `this` seems to fix https://github.com/TurboWarp/scratch-vm/issues/273.