Empty/missing node-sizing SYSTEM_RESERVED_ES parameter can result in kubelet not starting

We experienced two separate upgrade failures relating to the introduction of the SYSTEM_RESERVED_ES node sizing parameter, causing kubelet to stop running.

One cluster (clusterA) upgraded from 4.11.14 to 4.11.17. It experienced an issue whereby 
   /etc/node-sizing.env 
on its master nodes contained an empty SYSTEM_RESERVED_ES value:

---
cat /etc/node-sizing.env 
SYSTEM_RESERVED_MEMORY=5.36Gi
SYSTEM_RESERVED_CPU=0.11
SYSTEM_RESERVED_ES=
---

causing the kubelet to not start up. To restore service, this file was manually updated to set a value (1Gi), and kubelet was restarted.

We are uncertain what conditions led to this occuring on the clusterA master nodes as part of the upgrade.

A second cluster (clusterB) upgraded from 4.11.16 to 4.11.17. It experienced an issue whereby worker nodes were impacted by a similar problem, however this was because a custom node-sizing-enabled.env MachineConfig which did not set SYSTEM_RESERVED_ES

This caused existing worker nodes to go into a NotReady state after the ugprade, and additionally new nodes did not join the cluster as their kubelet would become impacted. 

For clusterB the conditions are more well-known of why the value is empty.

However, for both clusters, if SYSTEM_RESERVED_ES ends up as empty on a node it can cause the kubelet to not start. 

We have some asks as a result:
- Can MCO be made to recover from this situation if it occurs, perhaps  through application of a safe default if none exists, such that kubelet would start correctly?
- Can there possibly be alerting that could indicate and draw attention to the misconfiguration?

4.11.17

Have not been able to reproduce it on a fresh cluster upgrading from 4.11.16 to 4.11.17

If SYSTEM_RESERVED_ES is empty in /etc/node-sizing*env then a default should be applied and/or kubelet able to continue running.