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

Rust's str::split() on an empty string yields one empty-string element,
causing string_to_array('', ',') to return [''] instead of the correct
empty array []. Added an is_empty() guard in both the plain and
null_value variants of string_to_array_impl, and added a sqllogictest
case to cover this edge case.

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

Author: dd-david-levin

datafusion/functions-nested/src/string.rs

@@ -742,10 +742,15 @@ where
                             list_builder.append(true);
                         }
                         (Some(string), Some(delimiter)) => {
-                            string.split(delimiter).for_each(|s| {
-                                list_builder.values().append_value(s);
-                            });
-                            list_builder.append(true);
+                            if string.is_empty() {
+                                // empty string returns empty array (PostgreSQL compatibility)
+                                list_builder.append(true);
+                            } else {
+                                string.split(delimiter).for_each(|s| {
+                                    list_builder.values().append_value(s);
+                                });
+                                list_builder.append(true);
+                            }
                         }
                         (Some(string), None) => {
                             string.chars().map(|c| c.to_string()).for_each(|c| {
@@ -773,14 +778,19 @@ where
                         list_builder.append(true);
                     }
                     (Some(string), Some(delimiter)) => {
-                        string.split(delimiter).for_each(|s| {
-                            if Some(s) == null_value {
-                                list_builder.values().append_null();
-                            } else {
-                                list_builder.values().append_value(s);
-                            }
-                        });
-                        list_builder.append(true);
+                        if string.is_empty() {
+                            // empty string returns empty array (PostgreSQL compatibility)
+                            list_builder.append(true);
+                        } else {
+                            string.split(delimiter).for_each(|s| {
+                                if Some(s) == null_value {
+                                    list_builder.values().append_null();
+                                } else {
+                                    list_builder.values().append_value(s);
+                                }
+                            });
+                            list_builder.append(true);
+                        }
                     }
                     (Some(string), None) => {
                         string.chars().map(|c| c.to_string()).for_each(|c| {

datafusion/sqllogictest/test_files/array.slt

@@ -8623,6 +8623,12 @@ SELECT string_to_array('abcxxxdef', 'xxx')
 ----
 [abc, def]
 
+# empty string input should return empty array (PostgreSQL compatibility)
+query ?
+SELECT string_to_array('', ',')
+----
+[]
+
 query ?
 SELECT string_to_array('abc', '')
 ----