add an ignore_nulls option to json_encode for the pl.Struct column

[nikhilramolla]

Description

pl.Struct contains a json_encode method which dumps a struct to a json string.

It would to useful to skip null values in the struct's items when dumping.

Example:

df = pl.DataFrame(
    {
        "struct": [
            { "a": "p", "b": None },
            { "a": None, "b": "q" },
        ],
    },
    schema=[
        ("struct", pl.Struct({
            "a": pl.String,
            "b": pl.String,
        }))
    ],
)
print(df)
print(df.select(pl.col("struct").struct.json_encode()))
print(df.select(pl.col("struct").struct.json_encode(ignore_nulls=True)))

shape: (2, 1)
┌────────────┐
│ struct     │
│ ---        │
│ struct[2]  │
╞════════════╡
│ {"p",null} │
│ {null,"q"} │
└────────────┘
shape: (2, 1)
┌────────────────────┐
│ struct             │
│ ---                │
│ str                │
╞════════════════════╡
│ {"a":"p","b":null} │
│ {"a":null,"b":"q"} │
└────────────────────┘
shape: (2, 1)
┌───────────┐
│ struct    │
│ ---       │
│ str       │
╞═══════════╡
│ {"a":"p"} │
│ {"b":"q"} │
└───────────┘