fix: string_to_array('', delim) now returns empty array for PostgreSQL compatibility

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dd-david-levin

datafusion/functions-nested/src/string.rs

@@ -738,7 +738,9 @@ where
                 |(string, delimiter)| {
                     match (string, delimiter) {
                         (Some(string), Some("")) => {
-                            list_builder.values().append_value(string);
+                            if !string.is_empty() {
+                                list_builder.values().append_value(string);
+                            }
                             list_builder.append(true);
                         }
                         (Some(string), Some(delimiter)) => {
@@ -770,10 +772,12 @@ where
             .for_each(|((string, delimiter), null_value)| {
                 match (string, delimiter) {
                     (Some(string), Some("")) => {
-                        if Some(string) == null_value {
-                            list_builder.values().append_null();
-                        } else {
-                            list_builder.values().append_value(string);
+                        if !string.is_empty() {
+                            if Some(string) == null_value {
+                                list_builder.values().append_null();
+                            } else {
+                                list_builder.values().append_value(string);
+                            }
                         }
                         list_builder.append(true);
                     }

datafusion/sqllogictest/test_files/array.slt

@@ -8629,6 +8629,11 @@ SELECT string_to_array('', ',')
 ----
 []
 
+query ?
+SELECT string_to_array('', '')
+----
+[]
+
 query ?
 SELECT string_to_array('abc', '')
 ----