bug: Unnecessary warning when using reference to secret for Loki output httpUser and httpPassword

### Describe the issue

Hi all,

Following the example found in the the fluent-operator helm chart values, I specified the httpUser and httpPassword as a reference to a secret. I faced a warning which I find misleading. The installation succeeded though, so this issue is of low priority.

```myuser``` and ```mypass``` are the default values set for ```fluentbit.output.loki.httpUser``` and ```fluentbit.output.loki.httpPassword``` respectively. Maybe ```null``` would work better as default values.

### To Reproduce

my-values.yaml file contains

```yaml
fluentbit:
output:
loki:
httpUser:
valueFrom:
secretKeyRef:
key: value
name: husersecret
httpPassword:
valueFrom:
secretKeyRef:
key: value
name: hpasssecret
```

and run ```helm install my-fluent-operator fluent/fluent-operator --values my-values.yaml```

you should see the following warning

```
level=INFO msg="warning: destination for fluent-operator.fluent-operator.fluentbit.output.loki.httpUser is a table. Ignoring non-table value (myuser)"
level=INFO msg="warning: destination for fluent-operator.fluent-operator.fluentbit.output.loki.httpPassword is a table. Ignoring non-table value (mypass)"
```

### Expected behavior

I would expect the installation to succeed without warnings.

### Your Environment

```markdown
- Fluent Operator version: 3.5.0
- Container Runtime: containerd://2.1.4-k3s1
- Operating system: Ubuntu 24.04.3 LTS
- Kernel version: 6.8.0-90-generic
```

### How did you install fluent operator?

helm

### Additional context

_No response_