make accountNo optional

[lukas-staab]

Summary

This PR makes the $accountNo parameter nullable with a default value on both DatevAccountLedgerData::addAccountsReceivableLedger() and addAccountsPayableLedger():

• public function addAccountsReceivableLedger(float $amount, string $accountNo, ...)

• public function addAccountsReceivableLedger(float $amount, ?string $accountNo = null, ...)

Motivation

accountNo (the Gegenkonto / contra account) is optional in the DATEV schema this library implements. In xsds/Belegverwaltung_online_ledger_import_v060.xsd:

<xsd:element name="accountNo" type="de:p10039" minOccurs="0" maxOccurs="1">
  <xsd:documentation>Das Element accountNo enthält die Kontonummer des Gegenkontos.</xsd:documentation>
</xsd:element>

minOccurs="0" means the contra account may be omitted — DATEV resolves it later via automatic account assignment (Kontierung) rather than requiring it at import time. This is a common real-world case: the exporting system records the
amount and booking text but deliberately leaves the contra account to be determined in DATEV (e.g. by Steuerberater)

Currently the required string $accountNo forces callers to pass a value the schema treats as optional, which throws ArgumentCountError for an otherwise valid ledger entry. Every other optional field in these methods already follows
the ?type $x = null convention, so this aligns the two required-looking parameters with both the XSD and the rest of the signature.

Changes

• $accountNo changed from string $accountNo to ?string $accountNo = null in addAccountsReceivableLedger() and addAccountsPayableLedger().

Backward compatibility

Fully backward compatible — existing callers that pass accountNo are unaffected; only the "omit it" case becomes possible. The value is already stored as-is into the ledger array and emitted (or skipped) accordingly, matching the
XSD's minOccurs="0".

[lukas-staab]

Hey @ms-aranes any plans to merge this PR soon? Right now this is the blocking feature why i cannot use your libary, and i would like to use and ship it :) If you have any questions or suggestions please let me know!