fix: check if raw_input is empty string

src/lsp.rs

@@ -195,6 +195,7 @@ impl Backend {
         // prevent deleting puncts before real pinyin input
         let real_offset = new_offset
             + raw_input
+                .and_then(utils::option_string)
                 .and_then(|rime_raw_input| new_input.borrow_pinyin().rfind(&rime_raw_input))
                 .unwrap_or(0);