scanpypi: ignore empty elements in package requirements

Depending on how setup.py reads requirements files empty elements can occur.
This patch takes care, that such elements will be ignored and don't crash
the scanpypi script.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Tested-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>