Bug 1477737 [wpt PR 12143] - [idlharness.js] Check idl_test setup func defined, a=testonly

Automatic update from web-platform-tests[idlharness.js] Check idl_test setup func defined (#12143)

Allows the function to be unspecified/null rather than a no-op func.
--

wpt-commits: 67a2edf00ac1e65c6336d3f47638c73c339015d7
wpt-pr: 12143

testing/web-platform/tests/resources/idlharness.js

@@ -3186,7 +3186,9 @@ function idl_test(srcs, deps, idl_setup_func, test_name) {
                 }
             })
             .then(function() {
+                if (idl_setup_func) {
                     return idl_setup_func(idl_array, t);
+                }
             })
             .then(function() { idl_array.test(); })
             .catch(function (reason) {