fix: array_size returns -1 instead of null for null input

[andygrove]

Which issue does this PR close?

Closes #4560.

Rationale for this change

array_size(NULL) should return NULL, but Comet returned -1.

array_size is RuntimeReplaceable and lowers to Size(child, legacySizeOfNull = false). With legacySizeOfNull = false, Spark returns NULL for a NULL input; with legacySizeOfNull = true it returns -1.

Comet's Size serde built the else branch of its CASE WHEN from the session conf SQLConf.get.legacySizeOfNull rather than from the expression's own legacySizeOfNull field. Since spark.sql.legacy.sizeOfNull defaults to true, the else literal became -1 even though array_size had explicitly set the expression flag to false. The query runs natively via CometProject, so it did not fall back: it just returned the wrong value.

What changes are included in this PR?

CometSize.convert now reads expr.legacySizeOfNull instead of SQLConf.get.legacySizeOfNull. This honors the flag baked into each Size instance, which already accounts for both the array_size override (always false) and the ANSI handling Spark applies when constructing the expression. The regular size and cardinality paths are unaffected because their expressions carry the resolved session value.

How are these changes tested?

Added a test in CometArrayExpressionSuite asserting array_size returns NULL for a NULL array input under both spark.sql.legacy.sizeOfNull=false and true. The existing size - respect to legacySizeOfNull test continues to pass, confirming the size path is unchanged.

[comphead]

Thanks @andygrove

[hsiang-c]

Thanks @andygrove