Join operations that can introduce nulls (leftJoin, rightJoin, etc) lead to null values FrameColumn and NPE on basic operations

[koperagen]

Repro:

val l = (0..10).toDataFrame {
    "id" from { it }
}
val r = (0..5).toDataFrame {
    "id" from { it }
    "frame" from {
        (0..10).toDataFrame {
            "col" from { it }
        }
    }
}
val res = l.leftJoin(r)
res.print()

Actual:
Throws exception at print.

Exception in thread "main" java.lang.NullPointerException: Parameter specified as non-null is null: method org.jetbrains.kotlinx.dataframe.DataFrameKt.getNrow, parameter <this>
	at org.jetbrains.kotlinx.dataframe.DataFrameKt.getNrow(DataFrame.kt)

Culprit is this unchecked cast that sneaks AnyFrame? values into FrameColumn (we expect no nulls there):

dataframe/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/api/join.kt

Line 235 in 1232fd1

ColumnKind.Frame -> DataColumn.createFrameColumn(srcColumn.name, columnValues.asList() as List<AnyFrame>)

Expected:
At this point i'd expected frame column to have 5 DataFrames from r, and remaining 5 rows filled with empty dataframes with same schema: col: Int, but 0 rows
In that ColumnKind.Frame branch we should replace all nulls with "null object" dataframe:

dataframe/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/DataFrame.kt

Line 61 in 06990d0

public fun empty(schema: DataFrameSchema): AnyFrame = schema.createEmptyDataFrame()

[Jolanrensen]

Interesting, the debug mode should have picked this up on TC, but we may not have had a test for it yet :)