message: Fix validation to avoid `undefined` class on fields
  The class name handling for validation inadvertently added `undefined` as a
  class when validation is not used. This rearranges the logic to avoid the issue.
  Fixes
diff: @@ -86,20 +86,13 @@ export default class Field extends React.PureComponent {
  prefixContainer = <span className="mx_Field_prefix">{prefix}</span>;
  }
  - let validClass;
  - if (onValidate) {
  - validClass = classNames({
  - mx_Field_valid: this.state.valid === true,
  - mx_Field_invalid: this.state.valid === false,
  - });
  - }
  -
  const fieldClasses = classNames("mx_Field", `mx_Field_${inputElement}`, {
  // If we have a prefix element, leave the label always at the top left and
  // don't animate it, as it looks a bit clunky and would add complexity to do
  // properly.
  mx_Field_labelAlwaysTopLeft: prefix,
  - [validClass]: true,
  + mx_Field_valid: onValidate && this.state.valid === true,
  + mx_Field_invalid: onValidate && this.state.valid === false,
  });
  // handle displaying feedback on validity
  