[FIX] account_ebics: camt.054 without account currency

[ecino]

camt.054 notifications (e.g. from Raiffeisen) contain neither Acct/Ccy nor Bal, so the currency lookup in _split_camt returns an empty list and the import crashes:

File "account_ebics/models/ebics_file.py", line 589, in _split_camt
    currency_code = stmt.xpath(
IndexError: list index out of range

This change:

• falls back on the currency of the entries (Ntry/Amt/@Ccy) when neither the account currency nor a balance is present. XPath returns nodes in document order, so Acct/Ccy and Bal still take precedence for camt.053;
• adds an error notification and skips the statement, instead of crashing, when no currency can be found at all.

[luc-demeyer]

@ecino
Thanks for this nice contribution.
I analyzed your PR and took a slightly different approach for this issue, cf. #181
Can you have a look and test this fix with your camt.054 file ?
I'll merge the code once I receive a positive feedback from you and forward port it to account_ebics for Odoo 19.0 and 20.0.