Clearing Event Definition Aggregation "Select Field" causes events to trigger spuriously

When creating or editing an Event Definition using Aggregation, if the "Select Field (Optional)" is temporarily populated and then cleared, it sets this field an empty string "" in this field, which changes the event behaviour from if that field had never been populated (field set to null).

This causes aggregations using count() == 0 to spuriously trigger, even though there are logs shown using the "replay search" button on the triggered event.

Details and screenshots can be seen in this forum thread: https://community.graylog.org/t/events-triggering-count-0-even-though-replay-search-shows-logs/34592

## Expected Behavior
Temporarily populating the "Select Field (Optional)" and then clearing it again should have the same behaviour as that when it has never been populated.

## Current Behavior
Temporarily populating the "Select Field (Optional)" and then clearing it again causes the event to detect zero logs even though there are some.

## Possible Solution

1. Change the Event Definition web form to set the field to null instead of an empty string "" when clearing the value, OR
2. Change the event processing logic to treat null or empty string "" in the same way (exclude field from algorithm)

## Steps to Reproduce (for bugs)
1. Create an event definition with a search query that reliably matches log entries
2. Add aggregation count() == 0 (DO NOT TOUCH THE "Select Field (Optional)" when defining)
3. Notice that the event correctly does NOT trigger (since there are logs and therefore count() is greater than 1)
4. Edit event definition to temporarily populate the "Select Field (Optional)" and then clear it again with the "X". Save the edited event definition.
5. Notice that the event now triggers even though there are logs and therefore count() should be greater than 1.
6. Use replay search on triggered event to show that there were indeed logs.

## Context
I discovered this when trying to create event definitions for "No logs received from host xyz".
I have worked around this by using curl to do a GET of the event definition, and then a PUT of the event definition with only the series field value changed from “” to null.

```
$ curl -u USERNAME:"PASSWORD" -H "Accept: application/json" -X GET "https://graylog.woods.am/api/events/definitions/6779f186970c084e8e0a8f38"

{"_scope":"DEFAULT","id":"6779f186970c084e8e0a8f38","title":"No logs from homeassistant.woods.am","description":"","updated_at":"2025-01-06T02:00:24.800Z","matched_at":"2025-01-05T09:51:26.801Z","priority":2,"alert":true,"config":{"type":"aggregation-v1","query":"source:\"homeassistant.woods.am\"","query_parameters":[],"filters":[],"streams":["000000000000000000000001"],"stream_categories":[],"group_by":[],"series":[{"type":"count","id":"count-","field":""}],"conditions":{"expression":{"expr":"==","left":{"expr":"number-ref","ref":"count-"},"right":{"expr":"number","value":0.0}}},"search_within_ms":300000,"execute_every_ms":300000,"use_cron_scheduling":false,"cron_expression":null,"cron_timezone":null,"event_limit":100},"field_spec":{"source":{"data_type":"string","providers":[{"type":"template-v1","template":"homeassistant.woods.am","require_values":false}]},"message":{"data_type":"string","providers":[{"type":"template-v1","template":"No logs received for >5 mins","require_values":false}]}},"key_spec":[],"notification_settings":{"grace_period_ms":3600000,"backlog_size":0},"notifications":[{"notification_id":"676f7274eccf2a0bc86dcd88","notification_parameters":null}],"storage":[{"type":"persist-to-streams-v1","streams":["000000000000000000000002"]}],"scheduler":null,"state":"ENABLED"}

$ curl -u USERNAME:"PASSWORD" -H "Content-Type: application/json" -H "X-Requested-By: cli" -X PUT -d '{"_scope":"DEFAULT","id":"6779f186970c084e8e0a8f38","title":"No logs from homeassistant.woods.am","description":"","updated_at":"2025-01-06T02:00:24.800Z","matched_at":"2025-01-06T02:02:34.993Z","priority":2,"alert":true,"config":{"type":"aggregation-v1","query":"source:\"homeassistant.woods.am\"","query_parameters":[],"filters":[],"streams":["000000000000000000000001"],"stream_categories":[],"group_by":[],"series":[{"type":"count","id":"count-","field":null}],"conditions":{"expression":{"expr":"==","left":{"expr":"number-ref","ref":"count-"},"right":{"expr":"number","value":0.0}}},"search_within_ms":300000,"execute_every_ms":300000,"use_cron_scheduling":false,"cron_expression":null,"cron_timezone":null,"event_limit":100},"field_spec":{"source":{"data_type":"string","providers":[{"type":"template-v1","template":"homeassistant.woods.am","require_values":false}]},"message":{"data_type":"string","providers":[{"type":"template-v1","template":"No logs received for >5 mins","require_values":false}]}},"key_spec":[],"notification_settings":{"grace_period_ms":3600000,"backlog_size":0},"notifications":[{"notification_id":"676f7274eccf2a0bc86dcd88","notification_parameters":null}],"storage":[{"type":"persist-to-streams-v1","streams":["000000000000000000000002"]}],"scheduler":null,"state":"ENABLED"}'
```

## Your Environment

* Graylog Version: 6.1.4+7528370, codename Noir
* Java Version: Eclipse Adoptium 17.0.13 on Linux 6.6.69-1-lts
* OpenSearch Version: Graylog Datanode 6.1.4+7528370
* MongoDB Version: v8.0.4
* Operating System: Arch Linux, Podman containers
* Browser version: Firefox Nightly 135.0a1 (2025-01-06) (64-bit)
* Containers:
graylog [Image=docker.io/graylog/graylog:6.1](https://hub.docker.com/r/graylog/graylog)
graylog-datanode [Image=docker.io/graylog/graylog-datanode:6.1](https://hub.docker.com/r/graylog/graylog-datanode)
mongodb [Image=docker.io/library/mongo:latest](https://hub.docker.com/r/library/mongo)