Handle missing resource config in node version helpers

[j1sk1ss]

Description

fetch_latest_conf can return nil when the referenced resource has already been deleted. get_nodes_ver and set_nodes_ver_and_nodes used the returned value immediately, which could raise an attempt to index a nil value.

This PR adds nil guards for that case. get_nodes_ver now returns nil when the latest resource config is unavailable, and set_nodes_ver_and_nodes exits without updating nodes when the resource no longer exists.

Which issue(s) this PR fixes:

Fixes #

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

Found this with Svace static analyzer