clean_header: Fix error handling for no-such-file case

The function cleanupFile should always return a single value (as opposed
to a tuple or list). In addition, if it encounters an error, it is
expected to return a value that evaluates to False. As it stands,
however, it returns (None, None) in certain error cases. Change this
function to return None, in those cases.

We previously saw the following error message, when we tried to run
clean_header.py on a non-existent file.

Traceback (most recent call last):
  File "clean_header.py", line 208, in <module>
    b.updateGitFiles()
  File "utils.py", line 164, in updateGitFiles
    self._writeFile(dst)
  File "utils.py", line 136, in _writeFile
    f.write(self.new_data[dst])
TypeError: expected a string or other character buffer object

(cherry picked from commit 6d6b4cedd16c3b4d6de114c622739fc5d52f4c57)

Bug: 128420573
Change-Id: Id1dfab71e7efdee14950520df69f2e35219ee353
Merged-In: I5f717dd1a4388f598f0fd4bfd5e6129017de9095