Autocli: error when empty YANG group and grouping-treeref=true

[olofhagsand]

If autocli.xml contains:

  <grouping-treeref>true</grouping-treeref>

and a YANG group is empty, or more specifically does not contain any configuration statements, then the CLI exits with:

CLIgen tree 'grouping--default--0--ietf-crypto-types--generate-csr-grouping' not found

[olofhagsand]

Example:

grouping pg5 {
    description "Empty, see https://github.com/clicon/clixon/issues/579";  
    action reset;
 }