[2.3.0] Checkmk Werk 16383 created: Handle empty operational status during interface inventory

[//]: # (werk v2) # Handle empty operational status during interface inventory key | value ---------- | --- date | 2024-03-14T13:48:35+00:00 version | 2.3.0b3 class | fix edition | cre component | checks level | 1 compatible | yes `inv_if.py` crashed on empty oper_status. This is fixed now as at least on some Cisco ASA/FirePower devices this value is empty.