Add if to skip if either borrow/repay is having 0 amount (#106)

When just withdraw collateral, we should skip the borrow calls instead
of calling borrow with 0 amount.
It looks like it was made in two scripts originally, but simple if in
these two scripts do the tricks, so keep it in same script is more gas
saving imo.

src/DeFiScripts.sol

@@ -252,13 +252,19 @@ contract CometSupplyMultipleAssetsAndBorrow {
         }
 
         for (uint256 i = 0; i < assets.length;) {
-            IERC20(assets[i]).forceApprove(comet, amounts[i]);
-            IComet(comet).supply(assets[i], amounts[i]);
+            if (amounts[i] > 0) {
+                IERC20(assets[i]).forceApprove(comet, amounts[i]);
+                IComet(comet).supply(assets[i], amounts[i]);
+            }
+
             unchecked {
                 ++i;
             }
         }
-        IComet(comet).withdraw(baseAsset, borrow);
+
+        if (borrow > 0) {
+            IComet(comet).withdraw(baseAsset, borrow);
+        }
     }
 }
 
@@ -273,10 +279,16 @@ contract CometRepayAndWithdrawMultipleAssets {
             revert DeFiScriptErrors.InvalidInput();
         }
 
-        IERC20(baseAsset).forceApprove(comet, repay);
-        IComet(comet).supply(baseAsset, repay);
+        if (repay > 0) {
+            IERC20(baseAsset).forceApprove(comet, repay);
+            IComet(comet).supply(baseAsset, repay);
+        }
+
         for (uint256 i = 0; i < assets.length;) {
-            IComet(comet).withdraw(assets[i], amounts[i]);
+            if (amounts[i] > 0) {
+                IComet(comet).withdraw(assets[i], amounts[i]);
+            }
+
             unchecked {
                 ++i;
             }