Be nice for slapos.libnetworkcache.

Convert buildout emptiness to Nones.

src/zc/buildout/networkcache.py

@@ -72,6 +72,9 @@ def download_network_cached(dir_url, cache_url, path, url, logger,
     directory_key = get_directory_key(url)
     url = os.path.basename(url)
 
+    if len(signature_certificate_list) == 0:
+        # convert [] into None in order to call nc nicely
+        signature_certificate_list = None
     try:
         nc = NetworkcacheClient(cache_url, dir_url,
             signature_certificate_list=signature_certificate_list)
@@ -119,6 +122,9 @@ def upload_network_cached(dir_url, cache_url, external_url, path, logger,
               urlmd5=hashlib.md5(external_url).hexdigest())
 
     f = open(path, 'r')
+    if not signature_private_key_file:
+        # convert '' into None in order to call nc nicely
+        signature_private_key_file = None
     try:
         nc = NetworkcacheClient(cache_url, dir_url,
             signature_private_key_file=signature_private_key_file)