This is a proposal fix for issue #5136
I first reproduced the issue and created two unit tests, test_add_hline_empty_subplots.py and test_hline_subplots_bug.py, that check if the bug is resolved. Then I implemented the fix (outlined below), which passed both of the new unit tests, and in the process, no old tests were broken.

Root cause

add_shape() is called with exclude_empty_subplots=True. For a fresh make_subplots figure, every subplot is flagged empty, so the h/v line shape is filtered out and never appears, even after traces are later added.

Fix overview

When add_hline/add_vline are invoked and the figure contains no traces (len(self.data)==0), the code now disables the exclude empty subplots logic before delegating to add_shape, allowing the shape to be created and later displayed once data is added.

Code PR

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the code generator and not the generated files.
• I have added tests or modified existing tests.
• For a new feature, I have added documentation examples (please see the doc checklist as well).
• I have added a CHANGELOG entry if changing anything substantial.
• For a new feature or a change in behavior, I have updated the relevant docstrings in the code.