Permit the target browser to be an empty string.

Some driver configurations do not check the browser string (appium?),
but we still want to require it be set to something since it's part of
the wire protocol.

Fixes issue 8000.

javascript/node/selenium-webdriver/builder.js

@@ -272,9 +272,10 @@ Builder.prototype.build = function() {
 
   browser = capabilities.get(Capability.BROWSER_NAME);
 
-  if (!browser) {
-    throw Error(
-        'Target browser not defined; did you forget to call forBrowser()?');
+  if (typeof browser !== 'string') {
+    throw TypeError(
+        'Target browser must be a string, but is <' + (typeof browser) + '>;' +
+        ' did you forget to call forBrowser()?');
   }
 
   // Apply browser specific overrides.