Automatic merge from submit-queue (batch tested with PRs 42969, 42966)

kubeadm: fixed warning nil logging

**What this PR does / why we need it**: Fix bug in warning aggregation for preflight checks. Would cause logging like this:

`[preflight] WARNING: %!s(<nil>)`

Will now only append non-nil cases to warning.

**Special notes for your reviewer**: /cc @jbeda 

**Release note**:
```release-note
NONE
```