[FIX] l10n_ch: fix isr report crash with studio

Issue

    - Swiss company
    - Install Accounting & Studio
    - Journals > Customers Invoice
        - Communication standard: Switzerland
    - Journals > Bank
        - Swizz account with CHF ISR subscription number
    - Studio > Reports > ISR

    Traceback

Cause

    If the statement of the `l10n_ch_isr_optical_line` compute
    method is False, the value will be `None`.

    In the view, we do a foreach on the value which is `None` and
    it crashes.

Solution

    Set an empty string as default value for the
    `l10n_ch_isr_optical_line` field

OPW-2129122

closes odoo/odoo#41247

X-original-commit: a0dc556b0e04c9e89e7f6a8e4e9af045151614f7
Signed-off-by: Jason Van Malder <jasonvanmalder@users.noreply.github.com>

addons/l10n_ch/models/account_invoice.py

@@ -124,6 +124,7 @@ class AccountMove(models.Model):
             bank supporting the ISR (including the zeros).
         """
         for record in self:
+            record.l10n_ch_isr_optical_line = ''
             if record.l10n_ch_isr_number and record.l10n_ch_isr_subscription and record.currency_id.name:
                 #Left part
                 currency_code = None