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

@@ -2000,6 +2000,7 @@ def print_kernel_op_table(family, cw):
                             continue
                         dont_validate.append(x)
 
+                    if dont_validate:
                         members.append(('validate',
                                         ' | '.join([c_upper('genl-dont-validate-' + x)
                                                     for x in dont_validate])), )