Aggregating an empty DF does not make aggregation columns appear

Let's say we have

```kt
val df = dataFrameOf("a")("a")
val counted = df.drop(1).groupBy { a }.count()
```
or
```kt
val counted = df.drop(1).groupBy { a }.aggregate { count() into "count" }
```

In this case `counted` does not obtain the column `count`, it just has the empty column `a`.

This is an issue, because the compiler plugin cannot tell a dataframe is empty, so it will happily suggest `.count` and cause runtime exceptions:

We should at least get an empty `Int` column `count` in this case.

Thanks for the find @Allex-Nik!