fix: Improve null handling in array_to_string function (#18076)

* fix: Improve null handling in array_to_string function

* chore

Author: Weijun-H

datafusion/functions-nested/src/string.rs

@@ -369,41 +369,57 @@ pub(super) fn array_to_string_inner(args: &[ArrayRef]) -> Result<ArrayRef> {
             List(..) => {
                 let list_array = as_list_array(&arr)?;
                 for i in 0..list_array.len() {
-                    compute_array_to_string(
-                        arg,
-                        list_array.value(i),
-                        delimiter.clone(),
-                        null_string.clone(),
-                        with_null_string,
-                    )?;
+                    if !list_array.is_null(i) {
+                        compute_array_to_string(
+                            arg,
+                            list_array.value(i),
+                            delimiter.clone(),
+                            null_string.clone(),
+                            with_null_string,
+                        )?;
+                    } else if with_null_string {
+                        arg.push_str(&null_string);
+                        arg.push_str(&delimiter);
+                    }
                 }
 
                 Ok(arg)
             }
             FixedSizeList(..) => {
                 let list_array = as_fixed_size_list_array(&arr)?;
+
                 for i in 0..list_array.len() {
-                    compute_array_to_string(
-                        arg,
-                        list_array.value(i),
-                        delimiter.clone(),
-                        null_string.clone(),
-                        with_null_string,
-                    )?;
+                    if !list_array.is_null(i) {
+                        compute_array_to_string(
+                            arg,
+                            list_array.value(i),
+                            delimiter.clone(),
+                            null_string.clone(),
+                            with_null_string,
+                        )?;
+                    } else if with_null_string {
+                        arg.push_str(&null_string);
+                        arg.push_str(&delimiter);
+                    }
                 }
 
                 Ok(arg)
             }
             LargeList(..) => {
                 let list_array = as_large_list_array(&arr)?;
                 for i in 0..list_array.len() {
-                    compute_array_to_string(
-                        arg,
-                        list_array.value(i),
-                        delimiter.clone(),
-                        null_string.clone(),
-                        with_null_string,
-                    )?;
+                    if !list_array.is_null(i) {
+                        compute_array_to_string(
+                            arg,
+                            list_array.value(i),
+                            delimiter.clone(),
+                            null_string.clone(),
+                            with_null_string,
+                        )?;
+                    } else if with_null_string {
+                        arg.push_str(&null_string);
+                        arg.push_str(&delimiter);
+                    }
                 }
 
                 Ok(arg)

datafusion/sqllogictest/test_files/array.slt

@@ -4855,7 +4855,7 @@ h,-,-,-,o nil-2-nil-4-5 1|0|3
 query T
 select array_to_string(arrow_cast([arrow_cast([NULL, 'a'], 'FixedSizeList(2, Utf8)'), NULL], 'FixedSizeList(2, FixedSizeList(2, Utf8))'), ',', '-');
 ----
--,a,-,-
+-,a,-
 
 # array_to_string with columns #1