dfr count-null for dataframe

Counts null values.

Signature

> dfr count-null

Examples

Counts null values

> let s = ([1 1 0 0 3 3 4] | dfr into-df);
    ($s / $s) | dfr count-null
╭───┬────────────╮
│ # │ count_null │
├───┼────────────┤
│ 0 │          2 │
╰───┴────────────╯