refactor character_length impl by unifying null handling logic (#16877)

Author: waynexia

datafusion/functions/src/unicode/character_length.rs

@@ -136,56 +136,37 @@ where
     // string is ASCII only is relatively cheap.
     // If strings are ASCII only, count bytes instead.
     let is_array_ascii_only = array.is_ascii();
-    let array = if array.null_count() == 0 {
+    let nulls = array.nulls().cloned();
+    let array = {
         if is_array_ascii_only {
             let values: Vec<_> = (0..array.len())
                 .map(|i| {
-                    let value = array.value(i);
+                    // Safety: we are iterating with array.len() so the index is always valid
+                    let value = unsafe { array.value_unchecked(i) };
                     T::Native::usize_as(value.len())
                 })
                 .collect();
-            PrimitiveArray::<T>::new(values.into(), None)
+            PrimitiveArray::<T>::new(values.into(), nulls)
         } else {
             let values: Vec<_> = (0..array.len())
                 .map(|i| {
-                    let value = array.value(i);
-                    if value.is_ascii() {
-                        T::Native::usize_as(value.len())
+                    // Safety: we are iterating with array.len() so the index is always valid
+                    if array.is_null(i) {
+                        T::default_value()
                     } else {
-                        T::Native::usize_as(value.chars().count())
+                        let value = unsafe { array.value_unchecked(i) };
+                        if value.is_empty() {
+                            T::default_value()
+                        } else if value.is_ascii() {
+                            T::Native::usize_as(value.len())
+                        } else {
+                            T::Native::usize_as(value.chars().count())
+                        }
                     }
                 })
                 .collect();
-            PrimitiveArray::<T>::new(values.into(), None)
+            PrimitiveArray::<T>::new(values.into(), nulls)
         }
-    } else if is_array_ascii_only {
-        let values: Vec<_> = (0..array.len())
-            .map(|i| {
-                if array.is_null(i) {
-                    T::default_value()
-                } else {
-                    let value = array.value(i);
-                    T::Native::usize_as(value.len())
-                }
-            })
-            .collect();
-        PrimitiveArray::<T>::new(values.into(), array.nulls().cloned())
-    } else {
-        let values: Vec<_> = (0..array.len())
-            .map(|i| {
-                if array.is_null(i) {
-                    T::default_value()
-                } else {
-                    let value = array.value(i);
-                    if value.is_ascii() {
-                        T::Native::usize_as(value.len())
-                    } else {
-                        T::Native::usize_as(value.chars().count())
-                    }
-                }
-            })
-            .collect();
-        PrimitiveArray::<T>::new(values.into(), array.nulls().cloned())
     };
 
     Ok(Arc::new(array))