Fix overriding BrowserType launch defaults

For example, the following now picks the default values:
  // assuming passed from a JavaScript test script
  executablePath: "",
  executablePath: null,
  executablePath: undefined,

Fixes #407

Author: inancgumus

common/browser_options.go

@@ -60,10 +60,11 @@ func (l *LaunchOptions) Parse(ctx context.Context, opts goja.Value) error { //no
 		o  = opts.ToObject(rt)
 	)
 	for _, k := range o.Keys() {
-		var (
-			err error
-			v   = o.Get(k)
-		)
+		v := o.Get(k)
+		if v.Export() == nil {
+			continue // don't override the defaults on `null``
+		}
+		var err error
 		switch k {
 		case "args":
 			err = exportOpt(rt, k, v, &l.Args)

common/browser_options_test.go

@@ -30,6 +30,23 @@ func TestBrowserLaunchOptionsParse(t *testing.T) {
 				}, lo)
 			},
 		},
+		"nulls": { // don't override the defaults on `null`
+			opts: map[string]any{
+				"env":               nil,
+				"headless":          nil,
+				"logCategoryFilter": nil,
+				"timeout":           nil,
+			},
+			assert: func(tb testing.TB, lo *LaunchOptions) {
+				tb.Helper()
+				assert.Equal(tb, &LaunchOptions{
+					Env:               make(map[string]string),
+					Headless:          true,
+					LogCategoryFilter: ".*",
+					Timeout:           DefaultTimeout,
+				}, lo)
+			},
+		},
 		"args": {
 			opts: map[string]any{
 				"args": []any{"browser-arg1='value1", "browser-arg2=value2", "browser-flag"},