Fix: handle NULL offset of NTH_VALUE window function (#12851)

Author: HuSen8891

datafusion/physical-expr/src/window/nth_value.rs

@@ -185,7 +185,7 @@ impl PartitionEvaluator for NthValueEvaluator {
                         // Negative index represents reverse direction.
                         (n_range >= reverse_index, true)
                     }
-                    Ordering::Equal => (true, false),
+                    Ordering::Equal => (false, false),
                 }
             }
         };

datafusion/sqllogictest/test_files/window.slt

@@ -4929,6 +4929,15 @@ SELECT v1, NTH_VALUE(v2, 0) OVER (PARTITION BY v1 ORDER BY v2) FROM t;
 2 NULL
 2 NULL
 
+query I
+SELECT NTH_VALUE(tt0.v1, NULL) OVER (PARTITION BY tt0.v2 ORDER BY tt0.v1) FROM t AS tt0;
+----
+NULL
+NULL
+NULL
+NULL
+NULL
+
 statement ok
 DROP TABLE t;