Avoid inserting final newlines in empty files

This matches the behavior described by the EditorConfig spec for its
`insert_final_newline` option:

> Editors must not insert newlines in empty files when saving those
> files, even if `insert_final_newline = true`.

Co-authored-by: Axlefublr <101342105+Axlefublr@users.noreply.github.com>

helix-term/tests/test/commands/write.rs

@@ -482,6 +482,21 @@ async fn test_write_insert_final_newline_added_if_missing() -> anyhow::Result<()
     Ok(())
 }
 
+#[tokio::test(flavor = "multi_thread")]
+async fn test_write_insert_final_newline_unchanged_if_empty() -> anyhow::Result<()> {
+    let mut file = tempfile::NamedTempFile::new()?;
+    let mut app = helpers::AppBuilder::new()
+        .with_file(file.path(), None)
+        .with_input_text("#[|]#")
+        .build()?;
+
+    test_key_sequence(&mut app, Some(":w<ret>"), None, false).await?;
+
+    helpers::assert_file_has_content(&mut file, "")?;
+
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread")]
 async fn test_write_insert_final_newline_unchanged_if_not_missing() -> anyhow::Result<()> {
     let mut file = tempfile::NamedTempFile::new()?;