fix: handle empty point with hyperkzg

Currently, hyperkzg does not work if the point is empty. This commit simply defaults to using a trivial point. This default respects the commitment and evaluation.

src/provider/hyperkzg.rs

@@ -356,7 +356,11 @@ where
     point: &[E::Scalar],
     _eval: &E::Scalar,
   ) -> Result<Self::EvaluationArgument, NovaError> {
-    let x: Vec<E::Scalar> = point.to_vec();
+    let x: Vec<E::Scalar> = if point.is_empty() {
+      vec![E::Scalar::ZERO]
+    } else {
+      point.to_vec()
+    };
 
     //////////////// begin helper closures //////////
     let kzg_open = |f: &[E::Scalar], u: E::Scalar| -> G1Affine<E> {
@@ -517,7 +521,11 @@ where
     P_of_x: &E::Scalar,
     pi: &Self::EvaluationArgument,
   ) -> Result<(), NovaError> {
-    let x = point.to_vec();
+    let x = if point.is_empty() {
+      vec![E::Scalar::ZERO]
+    } else {
+      point.to_vec()
+    };
     let y = P_of_x;
 
     // vk is hashed in transcript already, so we do not add it here