import pandas as pd

columns = ["col1", "col2"]

left = pd.DataFrame([[1, 2]], columns=columns)
right = pd.DataFrame(columns=columns)

res = left.merge(right, on="col1", how="outer")
print(res)  # 空表没事儿

res = right.merge(left, on="col1", how="outer")
print(res)  # 空表没事儿