Avoid creating empty DEBIAN/symbols files during build.

Avoid creating empty DEBIAN/symbols files during build.

Once invoked, dpkg-gensymbols used to always write out a file
even if the scan didn't find any symbol information. Now, unless
the output file has been manually specified, it won't do so
anymore. With this change, it's now possible to always add
a dpkg-gensymbols call in debian/rules and it will really be a
no-op if the package doesn't contain a library.