Nothing cols fixes

[Jolanrensen]

This PR contains a handful of fixes and optimizations regarding Nothing(?) in column types:

• Fixes DataFrame with Nothing? column is detected as FormattedFrame<*>? in notebooks #1546 and adds a helpful note to colsOf to prevent future confusion
• DataColumn.empty() created a Unit-typed column. I have no idea why, I made it Nothing, which better represents that emptyCol.get(0): Nothing will always throw an exception. Added DataColumn.emptyOf<T>() if you need a specifically typed empty column.
• #1546 also discovered that exceptions in update provided little info, so I added UpdateException for this.
• While inspecting Column DataCollector for null-filled or empty columns, I rewrote ::class-logic to typeOf's (for efficiency), and added redundancy checks:
  • (If the data collector should produce a Nothing column, it can only do that if it has no values; so the resulting column must be empty)
  • (if the data collector should produce a Nothing? column, it can only do that if all its values are null)

I also checked usages of colsOf() querying for nullable types to see if similar issues like #1546 existed. Fortunately, in most cases, including null-filled columns are not an issue! All statistics functions simply skip nulls, for instance.

There are some unfortunate cases, like:

dataFrameOf(
    "a" to columnOf(1, 2, 3, null),
    "b" to columnOf(null, null, null, null),
    "c" to columnOf(7, 3, 2, 65),
).fillNulls { colsOf<Int?>() }.with { 0 }

which throw java.lang.IllegalArgumentException: Can not add value of class kotlin.Int to column of type kotlin.Nothing?. Value = 0 because column b: Nothing? is included by colsOf<Int?>(). However, hopefully, the note to add .filter { !it.allNulls() } is found :). Alternatively, we could decide to add an argument colsOf<Int?>(exactType = true) in the future.

[AndreiKingsley]

Great! I've missed that behavior a few times.

[AndreiKingsley]

Why colsOf<T?> except colsOf<Nothing> is not an option here?

[Jolanrensen]

Nothing cannot be used as reified argument

[Jolanrensen]

just have a look what we have to do to achieve typeOf<Nothing>() in dataframe XD

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

Line 563 in fc5033c

internal val nothingType: KType = typeOf<List<Nothing>>().arguments.first().type!!

[Jolanrensen]

we would need to have a shortcut for that nothingCols() for instance, but I'm not sure it's common enough

[Jolanrensen]

or, you know, they type

colsOf<T?>() except colsOf(typeOf<List<Nothing>>().arguments.first().type!!)

you know, elegant