Empty or missing Vault Agent token files are not re-read

If the specified `vault_agent_token_file` is empty or missing, it will not be re-read after being populated. This issue was introduced in version `0.29.3`.

We are able to workaround this issue by pre-populating the token file with a placeholder value.

### Consul Template version

Reproducible with `0.39.1` and presumably any version after `0.29.3`. Version `0.29.2` (and earlier) will re-read the token file successfully.

### Configuration

This config has generous retry attempts to make it easier to test. Versions less than `0.29.3` will successfully re-read the token file during a retry attempt and successfully render templates.

```hcl
vault {
address = "http://127.0.0.1:8200"
vault_agent_token_file = "./token"
retry {
enabled = true
attempts = 20
backoff = "3s"
max_backoff = "3s"
}
}

template {
source = "one.txt.ctmpl"
destination = "one.txt"
error_on_missing_key = true
backup = false
}
```

```liquid
{{- with secret "secret/one" }}{{ .Data.data.my_key }}{{- end }}
```

```liquid
{
"my_key": "my_value"
}
```

### Command

```shell
/consul-template-0.29.3 -log-level=debug -config config.hcl
```

### Debug output

[Version `0.39.1`](https://gist.github.com/twispt/9441f7ab71ec3c75bec6ef341dc9a15c) Vault token file was empty at the start and was populated at about retry 6

[Version `0.39.1` with placeholder token value](https://gist.github.com/twispt/35431656cf72ea3b902423be8e9f8bce) This time the token file contained a placeholder value at the start and succeeds after a valid token is set

[Version `0.29.2`](https://gist.github.com/twispt/92bc50cce62d6ec2d29507302319aef3) Vault token file was empty at the start

### Expected behavior

When a token file is created and populated during the Vault retry loop it should be re-read by the file watcher and used in subsequent requests.

### Actual behavior

All Vault retries are exhausted despite the token file containing a valid token.

### Steps to reproduce

1. Ensure the token file referenced in config is empty or missing
2. Start consul-template using the command above. Note failure and retry log messages
3. While retries are on-going populate the file with a valid token
4. `0.29.3` and up will exhaust all retries and fail, lesser versions will succeed on the next retry

### References

- https://github.com/hashicorp/consul-template/blob/main/CHANGELOG.md#v0293-sept-30-2022