[why?] Ignore empty value for 'download-cache' in 'buildout' section

Even though such configuration is wrong...

src/zc/buildout/buildout.py

@@ -421,6 +421,9 @@ class Buildout(DictMixin):
                 sectionkey = data['buildout'][name]
                 origdir = sectionkey.value
                 src = sectionkey.source
+                if not origdir:
+                    del data['buildout'][name]
+                    continue
                 if '${' in origdir:
                     continue
                 if not os.path.isabs(origdir):