Fix tokenizer so we don't destroy our textbuf writing an empty stack

* Also fix a bug in Argument

mwparserfromhell/nodes/argument.py

@@ -33,7 +33,7 @@ class Argument(Node):
     def __init__(self, name, default=None):
         super(Argument, self).__init__()
         self._name = name
-        self._default = None
+        self._default = default
 
     def __unicode__(self):
         start = "{{{" + str(self.name)

mwparserfromhell/parser/tokenizer.py

@@ -122,7 +122,8 @@ class Tokenizer(object):
         """Pop the current stack, write *text*, and then write the stack."""
         stack = self._pop()
         self._write_text(text)
-        self._write_all(stack)
+        if stack:
+            self._write_all(stack)
         self._head -= 1
 
     def _read(self, delta=0, wrap=False, strict=False):