[BUGFIX] Avoid "field doesn't have a default value" sql error

Creating new `ext:reactions` records are broken and emits an
sql error message. This happens when it's tried to switch to
the core-shipped type.

>> 2: SQL error: "Field 'fields' doesn't have a default value"

Using a default value on database level does not work with
all database backends TYPO3 v12 officially supports, which
has been tried with #100088 and mitigated with #100101. So
that's not an option here.

This change bakes the default value for `json` type fields
using TCA default setting again, which has been the way the
original `type=user dbType=JSON` variant used with #99226.

Resolves: #100154
Related: #100101
Related: #100088
Related: #99226
Related: #99062
Releases: main
Change-Id: I07533f36cfbc1f627b14727195bbcbd5c4b1e35c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78107


Tested-by: Frank Nägler <frank.naegler@typo3.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
...