Remove parse_vec_expr helper

Behavior of `parse_vec_expr` and `parse_exprs` is almost similar -- both
take a collection of expressions to parse. The only difference is that
`parse_vec_expr` returns `Option::None` when collections is empty, but
this difference in behavior does not correspond to difference in
function names. Since the function is used once only, remove it instead
of coming up with a fancy name.

datafusion/proto/src/logical_plan/from_proto.rs

@@ -586,7 +586,10 @@ pub fn parse_expr(
                 parse_exprs(&pb.args, registry, codec)?,
                 pb.distinct,
                 parse_optional_expr(pb.filter.as_deref(), registry, codec)?.map(Box::new),
-                parse_vec_expr(&pb.order_by, registry, codec)?,
+                match pb.order_by.len() {
+                    0 => None,
+                    _ => Some(parse_exprs(&pb.order_by, registry, codec)?),
+                },
                 None,
             )))
         }
@@ -676,16 +679,6 @@ pub fn from_proto_binary_op(op: &str) -> Result<Operator, Error> {
     }
 }
 
-fn parse_vec_expr(
-    p: &[protobuf::LogicalExprNode],
-    registry: &dyn FunctionRegistry,
-    codec: &dyn LogicalExtensionCodec,
-) -> Result<Option<Vec<Expr>>, Error> {
-    let res = parse_exprs(p, registry, codec)?;
-    // Convert empty vector to None.
-    Ok((!res.is_empty()).then_some(res))
-}
-
 fn parse_optional_expr(
     p: Option<&protobuf::LogicalExprNode>,
     registry: &dyn FunctionRegistry,