Unknown blocks for BE to JE should be air

[ElvinStarry]

Some cheaters leave many, many Education Edition blocks (e.g. underwater_torch ) around the world of my server years ago, and finding and pruning all of them is impossible for a years-old save that has 1.2GB.

Literally copying block ID may cause an assertion error for non-JE and non-BE blocks, so they should be air when converted.
Simple patch, not tested, hope it will work.

[kbinani]

Thank you for the PR.

One thing I'm concerned about is how the converter behaves when the output-supported version is older than the input world's version. For example, when je2be-desktop supports Java Edition up to 1.21.4 but processes a Bedrock 1.21.10 world, there can be blocks newly added between 1.21.5 and 1.21.10.
Under the changes proposed in this PR, those unknown new blocks would also be converted to air.

At the moment, I think there are several possible approaches:

1. Replace all unknown blocks with air (as in this PR)
2. Have the converter maintain an explicit list of Education Edition–only blocks and replace only those with air
3. Keep unknown blocks as their original names (the current behavior)

I haven't fully evaluated which approach is best yet, but I'd like to choose the option that maximizes the benefit for users.