`polars count-null` for dataframe

Counts null values.

Signature

> polars count-null {flags}

Input/output types:

input	output
polars_dataframe	polars_dataframe
polars_lazyframe	polars_lazyframe

Examples

Counts null values

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