configtxgen command fails to prompt empty certificate file

Since configtxgen failed to recognize and flag empty input certificate file while constructing the genesis block, the issue later prompted while starting the orderer. It would help/improve the usability if the command is updated to find the issue while generating the block.

Steps to recreate the issue:

```
1. Update the fabric-samples/test-network-nan-bash/configtx.yaml, with empty server.crt file
EtcdRaft:
Consenters:
- Host: 127.0.0.1
Port: 6050
ClientTLSCert: crypto-config/ordererOrganizations/example.com/orderers/orderer.example.com/tls/server.crt
ServerTLSCert: crypto-config/ordererOrganizations/example.com/orderers/orderer.example.com/tls/server.crt
2. create the system channel genesis block using configtxgen
3. start the orderer. It fails with following error
2022-11-24 05:55:07.304 PST 0063 ERRO [orderer.consensus.etcdraft] pemToDER -> Rejecting PEM block of server TLS cert for node 1, offending PEM is:
```