Invalid defaultGroupConjunction when using custom conjunctions

[oriolcx]

Describe the bug
Error happens when the config.conjuntions only contains custom conditions. If a group is created, the jsonLogicFormat() function throws an error.

To Reproduce
create a config:

const config ={
 ...initialConfig,
  conjunctions: {
    add: {
      label: '+',
      formatConj: (children, _conj, not) => (not ? 'NOT ' : '') + '(' + children.join(' + ') + ')',
      mongoConj: '$add',
      sqlFormatConj: (children, _conj, not) => (not ? 'NOT ' : '') + '(' + children.join(' + ') + ')',
      spelFormatConj: (children, _conj, not) => (not ? 'NOT ' : '') + '(' + children.join(' + ') + ')',
    },
  },
});

Then create a group in the query builder and call jsonLogicFormat() function.

Expected behavior
To not throw an error

Additional context
I found different options to fix this.

• One is defining in the settings the undocumented option to:
  defaultGroupConjunction: "add"

• An alternative, transparent to the user, would be to add in the function defaultGroupConjuntion in defaultUtils:

var defaultGroupConjunction = exports.defaultGroupConjunction = function defaultGroupConjunction(config) {
  var groupFieldConfig = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : null;
  groupFieldConfig = (0, _configUtils.getFieldConfig)(config, groupFieldConfig); // if `groupFieldConfig` is field name, not config
  var conjs = groupFieldConfig && groupFieldConfig.conjunctions || Object.keys(config.conjunctions);
  if (conjs.length == 1) return conjs[0];
  // Line to add
  if (conjs.length !== 0 && !conjs.includes(config.settings.defaultGroupConjunction)) return conjs[0];
  // End of line to add
  return config.settings.defaultGroupConjunction || config.settings.defaultConjunction || conjs[0];
};

Happy to create a pull request if this is of any help.