bug: datafusion-spark array_repeat incorrectly returns NULL when element is NULL

[andygrove]

Describe the bug

The datafusion-spark implementation of array_repeat incorrectly returns NULL when the first argument (element) is NULL. In Apache Spark, only a NULL count (second argument) produces a NULL result — a NULL element should be repeated into the array.

To Reproduce

PySpark (correct behavior):

SELECT array_repeat(NULL, 2);    -- [NULL, NULL]
SELECT array_repeat(NULL, 1);    -- [NULL]
SELECT array_repeat(NULL, 0);    -- []
SELECT array_repeat('x', NULL);  -- NULL

DataFusion-spark (incorrect behavior):

SELECT array_repeat(NULL, 2);    -- NULL (should be [NULL, NULL])
SELECT array_repeat(NULL, 1);    -- NULL (should be [NULL])
SELECT array_repeat(NULL, 0);    -- NULL (should be [])
SELECT array_repeat('x', NULL);  -- NULL (correct)

The .slt test at datafusion/sqllogictest/test_files/spark/array/array_repeat.slt line 59 has the wrong expected value (NULL instead of [NULL, NULL]). Line 79 also has a wrong expected value for the (NULL, 1) row (NULL instead of [NULL]).

Expected behavior

Expression	Spark result	datafusion-spark result
array_repeat('x', 3)	[x, x, x]	[x, x, x] ✓
array_repeat(NULL, 2)	[NULL, NULL]	NULL ✗
array_repeat(NULL, 1)	[NULL]	NULL ✗
array_repeat(NULL, 0)	[]	NULL ✗
array_repeat('x', NULL)	NULL	NULL ✓

Additional context

Root cause: SparkArrayRepeat::spark_array_repeat in datafusion/spark/src/function/array/repeat.rs uses compute_null_mask on all arguments, which returns NULL if any argument is NULL. But array_repeat should only return NULL when the count (second argument) is NULL — a NULL element should be passed through to DataFusion's underlying array_repeat, which correctly repeats it.

Fix: Only check the second argument (count) for NULL, not the first argument (element).

The .slt expected values at lines 59 and 79 will also need to be corrected.