refactor(contracts): internal M0 and Sherlock audit changes

contracts/src/NobleDollar.sol

@@ -85,7 +85,9 @@ contract NobleDollar is HypERC20 {
         }
     }
 
-    constructor(address mailbox_) HypERC20(6, 1, mailbox_) {}
+    constructor(address mailbox_) HypERC20(6, 1, mailbox_) {
+        _disableInitializers();
+    }
 
     function initialize(address hook_, address ism_) public virtual initializer {
         super.initialize("Noble Dollar", "USDN", hook_, ism_, msg.sender);
@@ -187,6 +189,9 @@ contract NobleDollar is HypERC20 {
         // Distribute yield, derive new index from the adjusted total supply.
         // NOTE: We don't want to perform any principal updates in the case of yield accrual.
         if (to == address(this)) {
+
+            if ($.totalPrincipal == 0) return;
+
             uint128 oldIndex = $.index;
 
             $.index = IndexingMath.getIndexRoundedDown(totalSupply(), $.totalPrincipal);