tools: ynl-gen: avoid rendering empty validate field

When dont-validate flags are filtered out for do/dump op, the list may
be empty. In that case, avoid rendering the validate field.

Fixes: fa8ba3502a ("ynl-gen-c.py: render netlink policies static for split ops")
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20230808090344.1368874-1-jiri@resnulli.us
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/ynl-gen-c.py

@@ -2000,9 +2000,10 @@ def print_kernel_op_table(family, cw):
                             continue
                         dont_validate.append(x)
 
-                    members.append(('validate',
-                                    ' | '.join([c_upper('genl-dont-validate-' + x)
-                                                for x in dont_validate])), )
+                    if dont_validate:
+                        members.append(('validate',
+                                        ' | '.join([c_upper('genl-dont-validate-' + x)
+                                                    for x in dont_validate])), )
                 name = c_lower(f"{family.name}-nl-{op_name}-{op_mode}it")
                 if 'pre' in op[op_mode]:
                     members.append((cb_names[op_mode]['pre'], c_lower(op[op_mode]['pre'])))