dpkg-gensymbols: fail when empty values are given to -v, -P or -e.

dpkg-gensymbols: fail when empty values are given to -v, -P or -e.

* scripts/dpkg-gensymbols.pl: Do not accept empty parameters for
-v, -P and -e. "-v" led to empty versions in symbols files and people used
that option by mistake believing it would enable a verbose mode.