`InputGroup` fails if `undefined` or `null` passed as child

[bedrich-schindler]

Following examples fail due to undefined or null as child of InputGroup

<InputGroup label="Your favourite fruit">
  <SelectField
    label="Your favourite fruit"
    onChange={(e) => setFruit(e.target.value)}
    options={options}
    value={fruit}
  />
  <TextField
    label="Variety"
    placeholder="Eg. Golden delicious"
  />
  {null /* or undefined */}
  <Button label="Submit" />
</InputGroup>

This part of implementation should be fixed:

const validationState = children.reduce(
  (state, child) => {
    if (state === 'invalid' || (state === 'warning' && child.props.validationState === 'valid')) {
      return state;
    }
    return child.props.validationState ?? state;
  },
  null,
);