Cannot recover after disabling all message processors

UI to enable message processors becomes non-functional after disabling all processors.

## Current Behavior

Disabled message processors cannot be enabled - the UI becomes unresponsive.
Reported in this [forum post](https://community.graylog.org/t/i-cant-enable-message-processors-configuration/35905).

## Steps to Reproduce (for bugs)

1. Open system/configurations/message processors
2. Uncheck all and save
3. Try to re-enable
4. Observe that UI spins indefinitely

Does not repro if disabling/enabling individual message processors. Only when you disable them all.

## Context

Basically a blocking bug, since the instance is broken once processors are disabled.
Workaround is to re-enable in MongoDB.

## Your Environment

* Graylog Version: 6.2.4, 6.3.0-rc2