Do not update last used check number with non-integer values

[agritheory]

https://github.com/agritheory/check_run/blob/version-13/check_run/overrides/payment_entry.py#L11

This would let a Payment Entry Reference (completed outside of Check Run ) like "Via ACH" to pass, which then cannot be incremented.

Fix should target both version-13 and version-14 branches.