[BUGFIX] Use `find()` to get option tags from select

Using `children()` to get the option tags from the select fields
will fail if the select field contains `optgroup` tags (e.g. rendered
by a hook).

This patch replaces `children()` with `find()` to cover this
edge case.

Resolves: #76097
Releases: master, 7.6
Change-Id: Ib80f422d465d9e48232b1820e524a01a97eb94e6
Reviewed-on: https://review.typo3.org/48044


Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>