SNOW-1335123 Fix DF Alias in the JOIN Condition

[sfc-gh-bli]

Fixed two errors

1. df alias doesn't work in the JOIN condition, for example df1.alias("a").join(df2.alias("b"), col("a.col") === col("b.col"))
2. df alias requires child's output, but the child's SnowflakePlan can be empty.