fix: Fix StoredUserConfig handling null subsections

fix: Fix StoredUserConfig handling null subsections

Te `StoredUserConfig` did not handle sections without a subsection.
When the subsection did not exist, i.e. was `null`, then the subsection
name would be set to the string "null". This is not how the config file
format works. It should create a `[SECTIONNAME]` entry instead.

This fix handles a `null` subsection correctly, by handling it as a
section without a subsection.