Trivial Description
`df.show` handle null should print NULL instead of null to consistent behavior;
Like as the following behavior is currently inconsistent: ``` shell scala> spark.sql("select decode(6, 1, 'Southlake', 2, 'San Francisco', 3, 'New Jersey', 4, 'Seattle') as result").show(false) +------+ |result| +------+ |null | +------+ ``` ``` shell spark-sql> DESC FUNCTION EXTENDED decode; function_desc Function: decode Class: org.apache.spark.sql.catalyst.expressions.Decode Usage: decode(bin, charset) - Decodes the first argument using the second argument character set. decode(expr, search, result [, search, result ] ... [, default]) - Compares expr to each search value in order. If expr is equal to a search value, decode returns the corresponding result. If no match is found, then it returns default. If default is omitted, it returns null. Extended Usage: Examples: > SELECT decode(encode('abc', 'utf-8'), 'utf-8'); abc > SELECT decode(2, 1, 'Southlake', 2, 'San Francisco', 3, 'New Jersey', 4, 'Seattle', 'Non domestic'); San Francisco > SELECT decode(6, 1, 'Southlake', 2, 'San Francisco', 3, 'New Jersey', 4, 'Seattle', 'Non domestic'); Non domestic > SELECT decode(6, 1, 'Southlake', 2, 'San Francisco', 3, 'New Jersey', 4, 'Seattle'); NULL Since: 3.2.0 Time taken: 0.074 seconds, Fetched 4 row(s) ``` ``` shell spark-sql> select decode(6, 1, 'Southlake', 2, 'San Francisco', 3, 'New Jersey', 4, 'Seattle'); NULL
