slapproxy: Support keys with NULL in slave instance

Just like reference implementation support empty keys.

/cc @rafael 

This makes slapproxy behave the same way as SlapOS Master for empty keys while requesting slaves.

/reviewed-on nexedi/slapos.core!96

slapos/proxy/views.py

@@ -689,7 +689,6 @@ def requestSlave(software_release, software_type, partition_reference, partition
   new_slave['slave_reference'] = slave_reference
 
   for key in partition_parameter_kw:
-    if partition_parameter_kw[key] is not None:
     new_slave[key] = partition_parameter_kw[key]
 
   # Add slave to partition slave_list if not present else replace information