Fix concat_elements_utf8view capacity initialization. (#18003)

## Which issue does this PR close?

- Relates to #17857 (See
#17857 (comment))

## Rationale for this change

The capacity calculation replaced with `left.len()` (assuming
`left.len()` and `right.len()` are the same). As the `with_capacity`
refers to the length of the views (or strings), not to the length of the
bytes

## Are these changes tested?

The function is already covered by tests.

## Are there any user-facing changes?
No

Author: samueleresca

datafusion/physical-expr/src/expressions/binary/kernels.rs

@@ -145,12 +145,14 @@ pub fn concat_elements_utf8view(
     left: &StringViewArray,
     right: &StringViewArray,
 ) -> std::result::Result<StringViewArray, ArrowError> {
-    let capacity = left
-        .data_buffers()
-        .iter()
-        .zip(right.data_buffers().iter())
-        .map(|(b1, b2)| b1.len() + b2.len())
-        .sum();
+    if left.len() != right.len() {
+        return Err(ArrowError::ComputeError(format!(
+            "Arrays must have the same length: {} != {}",
+            left.len(),
+            right.len()
+        )));
+    }
+    let capacity = left.len();
     let mut result = StringViewBuilder::with_capacity(capacity);
 
     // Avoid reallocations by writing to a reused buffer (note we