feat: ensure trailing newline with comment-only queries

Author: snrsw

src/sqlfmt/query.py

@@ -32,5 +32,8 @@ def nodes(self) -> List[Node]:
         return nodes
 
     def __str__(self) -> str:
+        if not self.lines:
+            return ""
+
         draft = [line.render_with_comments(self.line_length) for line in self.lines]
-        return "".join(draft)
+        return "".join(draft).rstrip("\n") + "\n"

tests/unit_tests/test_query.py

@@ -6,3 +6,10 @@ def test_whitespace_formatting(default_analyzer: Analyzer) -> None:
     expected_string = "select 1\nfrom my_table\nwhere true\n"
     q = default_analyzer.parse_query(source_string=source_string)
     assert str(q) == expected_string
+
+
+def test_only_comment_formatting(default_analyzer: Analyzer) -> None:
+    source_string = "-- a comment"
+    expected_string = "-- a comment\n"
+    q = default_analyzer.parse_query(source_string=source_string)
+    assert str(q) == expected_string