function assignMergeValue(object, key, value) {
  if (value !== undefined && !eq(object[key], value) || value === undefined && !(key in object)) {
    baseAssignValue(object, key, value);
  }
}
This function is like `assignValue` except that it doesn't assign
`undefined` values.

@private
@param {Object} object The object to modify.
@param {string} key The key of the property to assign.
@param {*} value The value to assign.