Add test to confirm SELECT isnan(NULL) works due to coercion with Exact

Author: shivbhatia10

datafusion/sqllogictest/test_files/spark/math/isnan.slt

@@ -83,3 +83,11 @@ SELECT
   isnan(CAST(NULL AS INT)) AS null_int;
 ----
 false false false false
+
+# Untyped NULL literal: relies on DataFusion coercing Null -> Float64 to satisfy
+# the Exact(Float32)/Exact(Float64) signature. Without that coercion, planning
+# would fail and we'd diverge from Spark, which returns false here.
+query B
+SELECT isnan(NULL);
+----
+false