bitbake: siggen: Make calc_taskhash match get_taskhash for file checksums

The code in these two functions is meant to be equivlanet in behaviour
but isn't. Add in code to ensure files that don't exist are handled
consistently by both functions. Users did report being able to generate
tracebacks otherwise.

(Bitbake rev: 51e913e178a02bb603ddf874669e3ce54f90bd5d)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>