Protect against nameless HTML tags in tools/lib/template_parser.py.

Fixed IndexError when there is only zero or more whitespace characters
between < and >. (str.split() will return an empty list in this case, which
means there is no index 0.)

tools/lib/template_parser.py

@@ -69,7 +69,13 @@ def tokenize(text):
     while state.i < len(text):
         if looking_at_html_start():
             s = get_html_tag(text, state.i)
-            tag = s[1:-1].split()[0]
+            tag_parts = s[1:-1].split()
+
+            if not tag_parts:
+                raise TemplateParserException("Tag name missing")
+
+            tag = tag_parts[0]
+
             if is_special_html_tag(s, tag):
                 kind = 'html_special'
             elif s.endswith('/>'):

tools/tests/test_template_parser.py

@@ -127,6 +127,13 @@ class ParserTest(unittest.TestCase):
         '''
         self._assert_validate_error('Tag missing >', text=my_html)
 
+    def test_validate_empty_html_tag(self):
+        # type: () -> None
+        my_html = '''
+            < >
+        '''
+        self._assert_validate_error('Tag name missing', text=my_html)
+
     def test_code_blocks(self):
         # type: () -> None