Testing if field exists in omit_if doesn't work

**Describe the bug**
According to https://docs.cloud.google.com/logging/docs/view/logging-query-language#missing_fields testing for missing logging fields should work using `:*`. Unfortunately, this doesn't work in `omit_if`. `move_fields` processor accepts `field:*` as a filter, but doesn't work in the end.

**To Reproduce**
Steps to reproduce the behavior:
1. Start a GCE VM with image 'rocky-linux-9-v20260115'
2. Install Ops Agent version 'google-cloud-ops-agent-2.63.0-1.el9.x86_64.rpm'
3. Customize the agent with the following configuration and restart the agent
```
logging:
receivers:
test_log:
type: tcp
format: json
processors:
add_context_on_exception:
type: modify_fields
fields:
jsonPayload.service:
static_value: 'my-service'
omit_if: -jsonPayload.exception:*
service:
pipelines:
default_pipeline:
processors: [add_context_on_exception]
receivers: [test_log]
```

**Expected behavior**
This should add `jsonPayload.service` field populated with `my-service` value only when `jsonPayload.exception` field exists, but it doesn't.

**Environment (please complete the following information):**
- VM distro / OS: Rocky Linux 9.7
- Ops Agent version 2.63.0

**Additional context**
The workaround I found is to use any character in the filter which I would expect in the exception field to be found, for example letter 'a'. Of course this is just workaround and doesn't work in all case, for example when letter 'a' is missing from the exception.

```
logging:
receivers:
test_log:
type: tcp
format: json
processors:
add_context_on_exception:
type: modify_fields
fields:
jsonPayload.service:
static_value: 'my-service'
omit_if: -jsonPayload.exception:"a"
service:
pipelines:
default_pipeline:
processors: [add_context_on_exception]
receivers: [test_log]
```