[Bug] `DataFrame.toJson()` does not recognize array columns correctly

[Jolanrensen]

Related to #2045

DataFrame.readJsonStr("""[{"label":"record"},[123]]""")

turn into

       label      array
     String?  List<Int>
        ----       ----
  0   record        [ ]
  1     null      [123]

while the array-column detection of toJson() does not correctly mark array as an array-column; it assumes the values in the array column must be null when other columns have values.
So we get:

df.toJson()

[{"label":"record","array":[]},{"label":null,"array":[123]}]

Expected behavior is to have it return to its original state.

Should be solved together with #2048