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

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):
https://github.com/Kotlin/dataframe/blob/1232fd13637af2192023c1f2a1b19e1bf03f0d99/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/api/join.kt#L235

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: https://github.com/Kotlin/dataframe/blob/06990d03a0b5f24aa3bfd2754ee54c64e62eb4ce/core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/DataFrame.kt#L61

Start in core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/api/join.kt at the unchecked cast near line 235, then read the null-object DataFrame definition in core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/DataFrame.kt around line 61. Reproduce the issue with the supplied leftJoin example and print call. Done means FrameColumn values contain empty DataFrames with the existing schema instead of nulls, and the example prints without an NPE.