Fixes lower() applied to None string, and not empty string as changeset 506 has mistakenly fixed.
src.dir = src
lib.dir = lib
doc.dir = doc
build.dir = build
product.name = yuicompressor
version.number = 2.4.2
jar.name = ${product.name}-${version.number}.jar
dist.package.name = ${product.name}-${version.number}