fix: remove comments, simplify branches, add null_string empty-input test

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

Author: dd-david-levin

datafusion/functions-nested/src/string.rs

@@ -744,23 +744,20 @@ where
                             list_builder.append(true);
                         }
                         (Some(string), Some(delimiter)) => {
-                            if string.is_empty() {
-                                // empty string returns empty array (PostgreSQL compatibility)
-                                list_builder.append(true);
-                            } else {
+                            if !string.is_empty() {
                                 string.split(delimiter).for_each(|s| {
                                     list_builder.values().append_value(s);
                                 });
-                                list_builder.append(true);
                             }
+                            list_builder.append(true);
                         }
                         (Some(string), None) => {
                             string.chars().map(|c| c.to_string()).for_each(|c| {
                                 list_builder.values().append_value(c.as_str());
                             });
                             list_builder.append(true);
                         }
-                        _ => list_builder.append(false), // null value
+                        _ => list_builder.append(false),
                     }
                 },
             )
@@ -782,19 +779,16 @@ where
                         list_builder.append(true);
                     }
                     (Some(string), Some(delimiter)) => {
-                        if string.is_empty() {
-                            // empty string returns empty array (PostgreSQL compatibility)
-                            list_builder.append(true);
-                        } else {
+                        if !string.is_empty() {
                             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);
                         }
+                        list_builder.append(true);
                     }
                     (Some(string), None) => {
                         string.chars().map(|c| c.to_string()).for_each(|c| {

datafusion/sqllogictest/test_files/array.slt

@@ -8634,6 +8634,13 @@ SELECT string_to_array('', '')
 ----
 []
 
+# empty string with null_string arg — without the fix "".split(",") yields [""],
+# which is != the null_string "x", so the result would be [""] instead of []
+query ?
+SELECT string_to_array('', ',', 'x')
+----
+[]
+
 query ?
 SELECT string_to_array('abc', '')
 ----