don't fail loading troubleshoot kinds if directory doesn't exist

[cbodonnell]

What this PR does / why we need it:

This PR modifies the LoadTSKindsFromPath function so that it will not fail if the path does not exist, but will instead return an empty TroubleshootKinds. This fixes an issue where re-running preflights for a sequence created on a KOTS version prior to 1.96.0 would hang indefinitely and log the error:

{"level":"error","ts":"2023-12-12T14:30:15Z","msg":"failed to run preflights: failed to load troubleshoot kinds from path: /tmp/kotsadm3762812014/rendered: failed to walk dir: /tmp/kotsadm3762812014/rendered: failed to walk dir: /tmp/kotsadm3762812014/rendered: lstat /tmp/kotsadm3762812014/rendered: no such file or directory"}

Which issue(s) this PR fixes:

https://app.shortcut.com/replicated/story/94221/re-running-preflights-for-versions-created-by-older-versions-of-kots-fails

Special notes for your reviewer:

Steps to reproduce

Does this PR introduce a user-facing change?

Fixes an issue where re-running preflights for sequences created by KOTS versions prior to 1.96.0 could hang indefinitely.

Does this PR require documentation?

NONE