multivpc. fix "NPE" in replace

[mr0re1]

Fix

╷
│ Error: Invalid function argument
│
│   on modules/embedded/modules/network/multivpc/main.tf line 19, in locals:
│   19:   network_name          = coalesce(replace(var.network_name_prefix, "_", "-"), replace(var.deployment_name, "_", "-"))
│     ├────────────────
│     │ while calling replace(str, substr, replace)
│     │ var.network_name_prefix is null
│
│ Invalid value for "str" parameter: argument must not be null.