[Bug] `DataFrame.readJson()` reads `null` as empty list in array column

```kt
DataFrame.readJsonStr("""[null, [123], []]""")
```
produces
```kt
         array
     List<Int>
          ----
  0        [ ]
  1      [123]
  2        [ ]
```
Expected:

```kt
         array
    List<Int>?
          ----
  0       null
  1      [123]
  2        [ ]
```

Fixes https://github.com/Kotlin/dataframe/issues/2046