fix: conditional set env rather than null value (#159)

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### Motivation

This is follow up of #157 

### Modifications

- Use conditional set env rather than null value, so we can leverage the
default value, otherwise null value can break it.

### Verifying this change

- [ ] Make sure that the change passes the CI checks.

*(Please pick either of the following options)*

This change is a trivial rework / code cleanup without any test
coverage.

*(or)*

This change is already covered by existing tests, such as *(please
describe tests)*.

*(or)*

This change added tests and can be verified as follows:

*(example:)*
- *Added integration tests for end-to-end deployment with large payloads
(10MB)*
  - *Extended integration test for recovery after broker failure*

### Documentation

- [x] `no-need-doc`

Signed-off-by: Max Xu <[email protected]>

Author: maxsxu

main.tf

@@ -285,11 +285,11 @@ module "eks" {
       most_recent    = true
       before_compute = true
       configuration_values = jsonencode({
-        env = {
+        env = merge(var.enable_vpc_cni_prefix_delegation ? {
           # Reference docs https://docs.aws.amazon.com/eks/latest/userguide/cni-increase-ip-addresses.html
-          ENABLE_PREFIX_DELEGATION = var.enable_vpc_cni_prefix_delegation ? "true" : null
-          WARM_PREFIX_TARGET       = var.enable_vpc_cni_prefix_delegation ? "1" : null
-        }
+          ENABLE_PREFIX_DELEGATION = "true"
+          WARM_PREFIX_TARGET       = "1"
+        } : {})
       })
     }
   }