Fix typo in empty tape store sentinel name

[luojiyin1987]

Summary

A private sentinel variable in src/bub/builtin/store.py is spelled _emtpy_store instead of _empty_store.

Impact

The typo does not currently break runtime behavior because the definition and references are consistent, but it does:

• reduce readability
• trigger typos warnings
• make the sentinel name harder to search for and maintain

Proposed fix

Rename _emtpy_store to _empty_store in the file and leave behavior unchanged.

Verification

• typos src/bub/builtin/store.py

[yihong0618]

human code!