Finding tagName with empty string should throw InvalidSelectorException

We have been returning a NoSuchElementException in single and [] in many
but "" leads to invalid CSS Selector which will be the standard way to
search for elements in the future.

javascript/atoms/locators/tag_name.js

@@ -17,6 +17,8 @@
 
 goog.provide('bot.locators.tagName');
 
+goog.require('bot.Error');
+goog.require('bot.ErrorCode');
 goog.require('goog.array');
 
 
@@ -29,6 +31,10 @@ goog.require('goog.array');
  *     such element could be found.
  */
 bot.locators.tagName.single = function(target, root) {
+  if (target === "") {
+    throw new bot.Error(bot.ErrorCode.INVALID_SELECTOR_ERROR,
+        'Unable to locate an element with the tagName ""');
+  }
   return root.getElementsByTagName(target)[0] || null;
 };
 
@@ -41,5 +47,9 @@ bot.locators.tagName.single = function(target, root) {
  * @return {goog.array.ArrayLike} All matching elements, or an empty list.
  */
 bot.locators.tagName.many = function(target, root) {
+  if (target === "") {
+    throw new bot.Error(bot.ErrorCode.INVALID_SELECTOR_ERROR,
+        'Unable to locate an element with the tagName ""');
+  }
   return root.getElementsByTagName(target);
 };

javascript/atoms/test/locator_test.html

@@ -323,6 +323,17 @@
       assertEquals(8, links.length);
     }
 
+    function testShouldThrowInvalidSelectorErrorWhenEmptyStringPassedIn() {
+      try {
+        bot.locators.findElements({tagName: ''});
+        fail('Should not have succeeded because the tagName is ""');
+      } catch (ex) {
+        // We expect an InvalidSelectorException because the xpath expression is
+        // syntactically invalid.
+        assertEquals(bot.ErrorCode.INVALID_SELECTOR_ERROR, ex.code);
+      }
+    }
+
     function testCanAddANewElementLocatingStrategy() {
       var expected = goog.dom.$('lion');
       bot.locators.add('fixed', {