Improve pgindent exclude handling: ignore empty lines

Hello,

We ran into an issue where pgindent stopped reformatting anything with
our custom exclude file, and after some investigation we found the
empty line accidentally inserted into the exclude file.

Pgindent currently treats empty lines as valid exclusions and creates
an empty regex from them. The empty regex matches any pattern,
resulting in ignoring everything.

As this behavior doesn't seem to be useful in practice, and it is easy
to reproduce accidentally (it works even at the end of the file), I
propose a patch that ignores empty lines in the exclude file.

If somebody really wants to ignore everything, that is still possible
with more explicit patterns like ".*"