Start with a fresh list in html2unicode every time

def x(..., something=[]):
    something.extend([1,2,3])

means something becomes [1,2,3] on the first call, but
[1,2,3,1,2,3] on the *second* call. This meant html2unicode
got a longer list of replacements every time it is called.
This commit changes it to the standard

def x(..., something=None):
   if something is None:
       something = []
which means it's always an empty list, instead of what's left from
the last call.

Change-Id: Ifb5221135498633e6e38967fcf4b5a02b14fb4d5
Compat-Change-Id: Ie490b575a8a0cc4b5d45bbb97c0606e0fd64d4f9
Merge "Start with a fresh list in html2unicode every time"