Uninitialized Files, File records or fields in a File record or File
usually contain a string of 0xff bytes. This becomes a problem when the
content is normally encoded/decoded as utf8 since by the construct
parser. The parser will throw an expection when it tries to decode the
0xff string as utf8. This is especially a serious problem in pySim-trace
where an execption stops the parser.
Let's fix this by interpreting a string of 0xff as an empty string.
Related: OS#6094
Change-Id: Id114096ccb8b7ff8fcc91e1ef3002526afa09cb7
791f80a44f