Antrea-native policy without a Tier set in it's spec should have "application" tier set

[abhiraut]

Describe the bug
When an Antrea-native policy is created without a Tier, it is pushed to the default "Application" Tier but the kubectl output shows the field as empty.

To Reproduce
Create ACNP without a Tier in spec.
kubectl get acnp <name> and notice Tier field is empty. Although the ACNP is by default associated to Application Tier.

Expected
Tier field is automatically set to "application" tier

Additional context
Achieve this by setting empty Tier filed to "application" in our mutation webhook.

[abhiraut]

note.. this bug has no effect on functionality .. more cosmetic