fix: Handle nil internal buffer (#2226)

#### Summary

`Bytes()` can panic if the internal `buf` is `nil`. This uses `Buf()` instead, see arrow code in https://github.com/apache/arrow-go/blob/b196d3b316d09f63786f021d4f1baa1fdd7620d2/arrow/memory/buffer.go#L108

---

Author: erezrokah

plugin/diff.go

@@ -20,12 +20,18 @@ func getUnifiedDiff(edits array.Edits, wantCol, haveCol arrow.Array) string {
 			wantDataType := wantCol.DataType()
 			wantData := make([]byte, wantCol.Len())
 			for _, buffer := range wantCol.Data().Buffers() {
-				wantData = append(wantData, buffer.Bytes()...)
+				buf := buffer.Buf()
+				if len(buf) > 0 {
+					wantData = append(wantData, buf...)
+				}
 			}
 			haveDataType := haveCol.DataType()
 			haveData := make([]byte, haveCol.Len())
 			for _, buffer := range haveCol.Data().Buffers() {
-				haveData = append(haveData, buffer.Bytes()...)
+				buf := buffer.Buf()
+				if len(buf) > 0 {
+					haveData = append(haveData, buf...)
+				}
 			}
 
 			wantBase64 := base64.StdEncoding.EncodeToString(wantData)