VPN: OpenVPN: Instances - fix "auth-gen-token" and "reneg-sec" value validation

[devopsoffice-ivan]

Allow saving config when reneg-sec is set to zero and auth-gen-token has no value, but prevents saving config when both parameters are set to zero.
Setting both parameters to zero results invalid configuration with error message:
Options error: --auth-gen-token needs a non-infinite --renegotiate_seconds setting

[fichtner]

Just for further reference this is related to #7690

[devopsoffice-ivan]

Yes, #7690 fixes a bad config value combination for reneg-sec and auth-gen-token (reneg-sec == 0 and auth-gen-token > 0) but blocks setting reneg-sec == 0 and empty auth-gen-token value.
Setting both reneg-sec and auth-gen-token to zero results an invalid configuration also, so an empty auth-gen-token value should be used when reneg-sec is set to zero. In this case auth-gen-token is skipped from generated configuration, just reneg-sec is included and the result is a valid generated configuration.
This patch allows setting empty value for auth-gen-token when reneg-sec is set to zero.