fix empty header error

[marin-jovanovic]

In case the file is empty header is empty.
This can is not handled and following exception is raised:

File "file_signatures.py", line 1493, in compare_sig
    if _byte != file_header[_loc]:
IndexError: list index out of range

This can be reproduced by calling
Matches(header=[])

This PR takes fixes that

[schlerp]

Excellent catch! When I wrote this library i had a very specific use case! Im glad others are finding it useful. Thanks for contributing :)

LGTM 🚀

Will be in next release which will be this week.