request recipe: Only append SLA parameter to the list if the key actually exists and is not empty string.

Conflicts:
	component/imagemagick/buildout.cfg
	component/libtiff/buildout.cfg
	component/postgresql/buildout.cfg
	setup.py

Conflicts:
	stack/lamp/buildout.cfg

All proposed solutions are worse than nothing.

This reverts commit 9e1711ab.

This reverts commit 8d8764bb.