lsp_python_diagnostic may use invalid lsprotocol.types.Position.character

[seeM]

Due to a bug in CPython (python/cpython#105911), the lsp_python_diagnostic function may encounter a value of err.offset = 0 (instead of the 1-indexed value guaranteed in the Python docs). If so, it will try to create an lsprotocol.types.Position with character=-1 causing a validation error.

I believe the caret would be at the correct position if we do something like err.offset = max(err.offset, 0) but I haven't tested that thoroughly.