Fix `isnan` function

File "/Users/s.singh/personal/spark-oss/python/pyspark/sql/connect/functions.py", line 288, in pyspark.sql.connect.functions.isnan
Failed example:
    df.select("a", "b", isnan("a").alias("r1"), isnan(df.b).alias("r2")).show()
Expected:
    +---+---+-----+-----+
    |  a|  b|   r1|   r2|
    +---+---+-----+-----+
    |1.0|NaN|false| true|
    |NaN|2.0| true|false|
    +---+---+-----+-----+
Got:
    +----+----+-----+-----+
    |   a|   b|   r1|   r2|
    +----+----+-----+-----+
    | 1.0|null|false|false|
    |null| 2.0|false|false|
    +----+----+-----+-----+
    <BLANKLINE>