MINOR: log: skip custom logformat_node name if empty

MINOR: log: skip custom logformat_node name if empty

Reminder:

Since 3.0-dev4, we can optionally give a name to logformat nodes:

  log-format "%(custom_name1)B %(custom_name2)[str(value)]"

But we may also optionally set the expected node type by appending
':type' after the name, type being either sint,str or bool, like this:

  log-format "%(string_as_int:sint)[str(14)]"

However, it is currently not possible to provide a type without providing
a name that is a least 1 char long. But it could be useful to provide a
type without setting a name, like this, for typecasting purposes only:

  log-format "%(:sint)[bool(true)]"

Thus in order to allow this usage, don't set node->name if node name is
not at least 1 character long. By doing so, node->name will remain NULL
and will not be considered, but the typecast setting will.