Panic when attempting to open empty document

[silvanshade]

This issue shows up in the ddlog-lsp/ddlog-lsp repo in the lsp::text_document::did_open test with the following:

---- lsp::text_document::did_open stdout ----
thread 'lsp::text_document::did_open' panicked at 'attempt to subtract with overflow', .../lsp-text-0.0.10/src/rope.rs:56:69
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace