The indent function strips empty lines that might be required

Describe the bug

The indent function strips empty lines that might be required. In our example the issue occurs while trying to print a PGP public key block. The empty line is required for the PGP PUBLIC KEY BLOCK to be considered valid.

To Reproduce

Create a secret on HashiCorp Vault with the following content

Create a ConfigMap file with the following content and replace the HashiCorp Vault path. (NOTE: the config key here is a multiline-string. Indentation in a yaml structure is not necessary since is already handled corretly by the plugin)
```
---
apiVersion: v1
kind: ConfigMap
data:
  config: |
    pgp_key: |
      <path:/mount_point/data/path/to/secret#key | indent 2>
```

Run the argocd-vault-plugin to template the file

# Version we have deployed
argocd-vault-plugin_1.17.0_linux_amd64 generate configmap.yaml

# Latest version
argocd-vault-plugin_1.18.1_linux_amd64 generate configmap.yaml

Expected behavior

We expect to have all content returned with proper indentation as below:

apiVersion: v1
data:
  config: |
    pgp_key: |
      -----BEGIN PGP PUBLIC KEY BLOCK-----

      TmV2ZXIgZ29ubmEgZ2l2ZSB5b3UgdXAKTmV2ZXIgZ29ubmEgbGV0IHlvdSBkb3duCk5ldmVyIGdv
      bm5hIHJ1biBhcm91bmQgYW5kIGRlc2VydCB5b3UKTmV2ZXIgZ29ubmEgbWFrZSB5b3UgY3J5Ck5l
      dmVyIGdvbm5hIHNheSBnb29kYnllCk5ldmVyIGdvbm5hIHRlbGwgYSBsaWUgYW5kIGh1cnQgeW91
      Cg==
      -----END PGP PUBLIC KEY BLOCK-----
kind: ConfigMap

Screenshots/Verbose output

We receive the output with the empty line removed by the indent plugin.

apiVersion: v1
data:
  config: |
    pgp_key: |
      -----BEGIN PGP PUBLIC KEY BLOCK-----
      TmV2ZXIgZ29ubmEgZ2l2ZSB5b3UgdXAKTmV2ZXIgZ29ubmEgbGV0IHlvdSBkb3duCk5ldmVyIGdv
      bm5hIHJ1biBhcm91bmQgYW5kIGRlc2VydCB5b3UKTmV2ZXIgZ29ubmEgbWFrZSB5b3UgY3J5Ck5l
      dmVyIGdvbm5hIHNheSBnb29kYnllCk5ldmVyIGdvbm5hIHRlbGwgYSBsaWUgYW5kIGh1cnQgeW91
      Cg==
      -----END PGP PUBLIC KEY BLOCK-----
kind: ConfigMap

Additional Infromation

I assume the problem comes from here: https://github.com/argoproj-labs/argocd-vault-plugin/blob/810f1fe9b4a064b145621ac42113860487551bd7/pkg/kube/modifiers.go#L49-L57

I think the function should always write the trailing newline and not only if the length of the line is greater than 0.

argoproj-labs / argocd-vault-plugin

The indent function strips empty lines that might be required #650