tools: ynl: don't return None for dumps

YNL currently reports None for empty dump:

 $ cli.py ...netdev.yaml --dump page-pool-get
 None

This doesn't matter for the CLI but when writing YNL based tests
having to deal with either list or None is annoying. Limit the
None conversion to non-dump ops:

 $ cli.py ...netdev.yaml --dump page-pool-get
 []
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240412141436.828666-3-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/lib/ynl.py

@@ -995,9 +995,11 @@ class YnlFamily(SpecFamily):
                     rsp_msg.update(self._decode_struct(decoded.raw, op.fixed_header))
                 rsp.append(rsp_msg)
 
+        if dump:
+            return rsp
         if not rsp:
             return None
-        if not dump and len(rsp) == 1:
+        if len(rsp) == 1:
             return rsp[0]
         return rsp