handle unknown language when parsing dependencies

wakatime/languages/__init__.py

@@ -42,8 +42,10 @@ class TokenParser(object):
         self._save_dependency(dep, truncate=truncate)
 
     def _extract_tokens(self):
+        if self.lexer:
             with open(self.source_file, 'r', encoding='utf-8') as fh:
                 return self.lexer.get_tokens_unprocessed(fh.read(512000))
+        return []
 
     def _save_dependency(self, dep, truncate=True):
         dep = dep.strip().split('.')[0].strip() if truncate else dep.strip()
@@ -60,6 +62,7 @@ class DependencyParser(object):
         self.source_file = source_file
         self.lexer = lexer
 
+        if self.lexer:
             try:
                 module_name = self.lexer.__module__.split('.')[-1]
                 class_name = self.lexer.__class__.__name__.replace('Lexer', 'Parser', 1)