fix(setup): skip AUXILIARY_VISION_MODEL write when input is blank

Guard the save_env_value('AUXILIARY_VISION_MODEL', ...) call with
'if _selected_vision_model:' so blank input at the non-OpenAI vision
model prompt doesn't nuke existing values in .env.

save_env_value has no internal guard against empty strings — it
faithfully writes whatever it receives, including empty values that
shadow the previously-configured model.

Salvage of #15504 (core hunk). Contributor's test was dropped because
it collided with subsequent test refactors; the fix stands on its own.

Co-authored-by: alt-glitch <balyan.sid@gmail.com>

hermes_cli/setup.py

@@ -964,6 +964,7 @@ def setup_model_provider(config: dict, *, quick: bool = False):
                     )
                 else:
                     _selected_vision_model = prompt("  Vision model (blank = use main/custom default)").strip()
+                if _selected_vision_model:
                     save_env_value("AUXILIARY_VISION_MODEL", _selected_vision_model)
                 print_success(
                     f"Vision configured with {_base_url}"