Fix acorn-optimizer error when destructuring array with empty elements (emscripten-core#21501)

This fixes cases like this:

function ([, a]) { }

In such arrays it is possible to have empty elements, which must be skipped.

Author: nsfisis

test/optimizer/JSDCE-objectPattern-output.js

@@ -2,7 +2,7 @@ let d = 40;
 
 let z = 50;
 
-globalThis.f = function(r) {
+globalThis.f = function([, r]) {
  let {a: a, b: b} = r;
  let {z: c} = r;
  let [, i, {foo: p, bar: q}] = r;

test/optimizer/JSDCE-objectPattern.js

@@ -10,7 +10,7 @@ let d = 40;
 // z.
 let z = 50;
 
-globalThis.f = function(r) {
+globalThis.f = function([/*empty*/, r]) {
   let { a, b } = r;
   let { z: c } = r;
   let [/*empty*/, i, {foo : p, bar : q}] = r;

tools/acorn-optimizer.mjs

@@ -388,7 +388,7 @@ function runJSDCE(ast, aggressive) {
         }
         if (param.type === 'ArrayPattern') {
           for (var elem of param.elements) {
-            traverse(elem);
+            if (elem) traverse(elem);
           }
         } else if (param.type === 'ObjectPattern') {
           for (var prop of param.properties) {