optional env deck variables

Currently all env variables need to be set when using deck, however it creates a problem with some cases and deck usages. Take this as an example:

plugins:
- name: acme
  config:
    account_email: ${{ env "DECK_ACME_ACCOUNT_EMAIL" }}
    api_uri: ${{ env "DECK_ACME_API_URI" }}
    eab_kid: ${{ env "DECK_ACME_EAB_KEY_ID" }}
    eab_hmac_key: ${{ env "DECK_ACME_EAB_SECRET" }}
    ${{- if env "DECK_ACME_ACCOUNT_KEY" }}
    account_key: ${{ env "DECK_ACME_ACCOUNT_KEY" }}
    ${{- end }}
    allow_any_domain: true
    tos_accepted: true
    storage: redis
    storage_config:
      redis:
        host: ${{ env "DECK_REDIS_HOST" }}
        port: 6379
        auth: ${{ env "DECK_REDIS_KEY" }}
        ssl: true
        ssl_verify: false

So we have potential cases where DECK_ACME_ACCOUNT_KEY is set, in the helm context when deploying the deck config. However by this if condition it makes the deck file merge unusable because it's not a valid yaml file.

What we can do is introducing envOptional to get rid of this problem. So envOptional would just output an empty string ''; in addition to this we can add also a nullFallback function to return null in case of empty string. So in this case would have a template like this:

plugins:
- name: acme
  config:
...
    account_key: ${{ envOptional "DECK_ACME_ACCOUNT_KEY" }}

will output

```yaml
plugins:
- name: acme
  config:
...
    account_key: ''

with nullFallback we would have this

plugins:
- name: acme
  config:
...
    account_key: ${{ envOptional "DECK_ACME_ACCOUNT_KEY" | nullFallback }}

will output

```yaml
plugins:
- name: acme
  config:
...
    account_key: null

And this would solve many of templating issues.

Start with the issue's proposed envOptional and nullFallback behavior, then locate decK's existing environment-variable template handling and its tests. Confirm how empty strings and null values are rendered and parsed, and consider both standalone optional values and pipeline usage; done means optional variables no longer require conditional YAML and the documented examples produce valid configuration.