[kernel] Add missing zero_ok changes

This change adds some changes I missed as part of the previous (see
da5c1e9) zero_ok change.

definitions/objects/system.def

@@ -15,7 +15,7 @@ object system : object {
 
     # Get a log line from the kernel log
     method get_log [cap:get_log] {
-        param buffer buffer [out optional]   # Buffer for the log message data structure
+        param buffer buffer [out zero_ok]   # Buffer for the log message data structure
     }
 
     # Ask the kernel to send this process messages whenever

definitions/syscalls.def

@@ -32,7 +32,7 @@ interface syscalls [syscall] {
     # supplied list is not big enough, will set the size
     # needed in `size` and return j6_err_insufficient
     function handle_list {
-        param handles ref object [list inout optional] # A list of handles to be filled
+        param handles ref object [list inout zero_ok] # A list of handles to be filled
     }
 
     # Create a clone of an existing handle, possibly with