fix(ocr): handle empty-string HTRMOPO_DIR env var with or-fallback

os.environ.get(key, default) returns "" when the key exists but is blank —
the default is only used when the key is absent. The or-fallback treats both
absence and blank values as "use the default".

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ocr-service/test_ensure_blla_model.py

@@ -29,6 +29,15 @@ def test_htrmopo_dir_default_is_fixed_path():
     assert result == "/app/models/.htrmopo"
 
 
+def test_htrmopo_dir_falls_back_to_default_when_set_to_empty_string():
+    """HTRMOPO_DIR='' must not produce an empty path — get() returns '' for blank env vars."""
+    with patch.dict(os.environ, {"HTRMOPO_DIR": ""}):
+        importlib.reload(ensure_blla_model)
+        result = ensure_blla_model.HTRMOPO_DIR
+    importlib.reload(ensure_blla_model)
+    assert result != ""
+
+
 # ─── Model already loadable ───────────────────────────────────────────────────