Handle people with legacy databases that have custom columns with non ascii search names. Fixes #781490 (KeyError happens whenever I click on a book in the main screen (list))

src/calibre/ebooks/metadata/book/base.py

@@ -112,10 +112,15 @@ class Metadata(object):
 
         Be careful with numeric fields since this will return True for zero as
         well as None.
+
+        Also returns True if the field does not exist.
         '''
+        try:
             null_val = NULL_VALUES.get(field, None)
             val = getattr(self, field, None)
             return not val or val == null_val
+        except:
+            return True
 
     def __getattribute__(self, field):
         _data = object.__getattribute__(self, '_data')