add defaults for the several structs

src/dynamics/solver/joint_constraint/joint_generic_velocity_constraint.rs

@@ -29,6 +29,12 @@ pub struct JointGenericVelocityConstraint {
     pub writeback_id: WritebackId,
 }
 
+impl Default for JointGenericVelocityConstraint {
+    fn default() -> Self {
+        JointGenericVelocityConstraint::invalid()
+    }
+}
+
 impl JointGenericVelocityConstraint {
     pub fn invalid() -> Self {
         Self {
@@ -313,6 +319,11 @@ pub struct JointGenericVelocityGroundConstraint {
 
     pub writeback_id: WritebackId,
 }
+impl Default for JointGenericVelocityGroundConstraint{
+    fn default() -> Self {
+        JointGenericVelocityGroundConstraint::invalid() 
+    }
+}
 
 impl JointGenericVelocityGroundConstraint {
     pub fn invalid() -> Self {