[aws-elasticloadbalancingv2] NLB does not respect crossZoneEnabled flag fully

__[Not a Contribution]__

NLB does not respect crossZoneEnabled flag fully when it's set to false or it's not set

### Reproduction Steps

1. Set `crossZoneEnabled` to `true` for a NLB
1. Run `cdk deploy`
1. Observe that cross-zone load balancing is enabled for this NLB
1. Set `crossZoneEnabled` to `false` for the same NLB
1. Run `cdk deploy`
1. Observe that cross-zone load balancing is __still enabled__ for this NLB

### Error Log

No error produced by CDK

### Environment

- **CLI Version :** 1.50.0 (build 84acc92)
- **Framework Version:**
- **Node.js Version:** v14.5.0
- **OS :** MacOS 10.15.5
- **Language (Version):** all

### Other

Add an `else` statement here to account for when `crossZoneEnabled = false`
https://github.com/aws/aws-cdk/blob/d6a126508e4bb03f6f9d874c2c6648c3e3661a41/packages/%40aws-cdk/aws-elasticloadbalancingv2/lib/nlb/network-load-balancer.ts#L91

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This is :bug: Bug Report

Start in packages/@aws-cdk/aws-elasticloadbalancingv2/lib/nlb/network-load-balancer.ts around line 91, then reproduce the reported toggle with cdk deploy. Done means changing crossZoneEnabled from true to false, or leaving it unset, causes the NLB's cross-zone load balancing setting to be disabled without a CDK error.

Written by the indexing model from the issue text.