Add test for empty SSE event

tests/test_sse.py

@@ -396,6 +396,10 @@ def test_format_sse_event_splitlines_behavior_in_comment():
     assert format_sse_event(comment="hi\n") == b": hi\n: \n\n"
 
 
+def test_format_sse_event_without_fields():
+    assert format_sse_event() == b"\n"
+
+
 # default_response_class tests