Kibana version:
7.8.0-SNAPSHOT (BC1)

Elasticsearch version:
7.8.0-SNAPSHOT (BC1)

Original install method (e.g. download page, yum, from source, etc.):
Cloud

Describe the bug:
Clearing the filter in the central management UI for a Metric Threshold Alert throws an error when saving.

Steps to reproduce:

1. Create a Metric Threshold Alert with a filter (either from Metrics Explorer or the central management)
2. Ensure alert is triggering
3. Edit alert in central management UI
4. Clear filter
5. Save (witness error)

Expected behavior:

It should save the alert without the filter

Screenshots (if relevant):