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

Rust's str::split() on an empty string always yields one empty-string
element, so "".split(",") produces [""]. The empty-delimiter branch
also unconditionally appended the (empty) string value. Both cases
now guard with !string.is_empty() to return a truly empty array,
matching PostgreSQL behavior.

Tests use cardinality() to unambiguously verify the result since
Arrow's text format renders [""] identically to [].

Author: dd-david-levin

datafusion/functions-nested/src/string.rs

@@ -738,13 +738,17 @@ 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)) => {
-                            string.split(delimiter).for_each(|s| {
-                                list_builder.values().append_value(s);
-                            });
+                            if !string.is_empty() {
+                                string.split(delimiter).for_each(|s| {
+                                    list_builder.values().append_value(s);
+                                });
+                            }
                             list_builder.append(true);
                         }
                         (Some(string), None) => {
@@ -753,7 +757,7 @@ where
                             });
                             list_builder.append(true);
                         }
-                        _ => list_builder.append(false), // null value
+                        _ => list_builder.append(false),
                     }
                 },
             )
@@ -765,21 +769,25 @@ 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);
                     }
                     (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);
-                            }
-                        });
+                        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);
                     }
                     (Some(string), None) => {

datafusion/sqllogictest/test_files/array.slt

@@ -8623,6 +8623,26 @@ SELECT string_to_array('abcxxxdef', 'xxx')
 ----
 [abc, def]
 
+query I
+SELECT cardinality(string_to_array('', ','))
+----
+0
+
+query I
+SELECT cardinality(string_to_array('', ''))
+----
+0
+
+query I
+SELECT cardinality(string_to_array('', ',', 'x'))
+----
+0
+
+query I
+SELECT cardinality(string_to_array('', '', 'x'))
+----
+0
+
 query ?
 SELECT string_to_array('abc', '')
 ----