[FIX] base: prevent json decoder error for default values

JSONDecoderError occurs when users enters invalid JSON format data in
'Default Value' field inside 'User-defined Defaults' and wherever this field is
being accessed to get default value this traceback will be generated.

Steps to reproduce:
1) Install 'Contacts' module.
2) Open 'Settings' > 'Technical' > 'User-defined Defaults'.
3) Click on record 'Language' > 'EDIT' button > in 'Default Value' field enter
   any improper JSON format data (e.g 'Maa' : FI ) .
4) Now, open 'Contacts' module > click on 'CREATE' button and traceback would be
   generated.

By applying this, it will check for proper JSON format.

Sentry-4169062951

closes odoo/odoo#126202

X-original-commit: 0e6e322d
Signed-off-by: Rémy Voet (ryv) <ryv@odoo.com>