[FIX] l10n_ch: Update l10n_ch_postal if related field is set

Fixes https://github.com/odoo/odoo/issues/43122


Before this commit, Creating Journal without Bank account number will be failed as, It tries to update Related field 'l10n_ch_postal' while bank_account_id is not set.

In this commit, we update 'l10n_ch_postal' if 'bank_account_id' is present on Journal

closes odoo/odoo#43295

X-original-commit: 6ddd93bf
Signed-off-by: oco-odoo <oco-odoo@users.noreply.github.com>