toBeEmpty( received, source ) {
      if ( typeof received !== "string" ) {
        return expectReturn( false,
          `[${ source }] expected ${ JSON.stringify( received ) } to be a string, but it is ${ typeof received }` );
      }
      const pass = !received.trim().length;
      return expectReturn( pass,
        `[${ source }] expected ${ JSON.stringify( received ) } to be empty`,
        `[${ source }] expected ${ JSON.stringify( received ) } to be not empty` );
    }
Assert value is an empty string
@param {String} received
@param {String} source
@returns {Object}