Implement Default for Buffer & ScalarBuffer (#7413)

* Implement `Default` for `ScalarBuffer`

* `impl Default for Buffer`

* Auto-derive Default

Author: emilk

arrow-buffer/src/buffer/immutable.rs

@@ -82,6 +82,13 @@ pub struct Buffer {
     length: usize,
 }
 
+impl Default for Buffer {
+    #[inline]
+    fn default() -> Self {
+        MutableBuffer::default().into()
+    }
+}
+
 impl PartialEq for Buffer {
     fn eq(&self, other: &Self) -> bool {
         self.as_slice().eq(other.as_slice())

arrow-buffer/src/buffer/scalar.rs

@@ -41,7 +41,7 @@ use std::ops::Deref;
 /// let sliced = buffer.slice(1, 2);
 /// assert_eq!(&sliced, &[2, 3]);
 /// ```
-#[derive(Clone)]
+#[derive(Clone, Default)]
 pub struct ScalarBuffer<T: ArrowNativeType> {
     /// Underlying data buffer
     buffer: Buffer,