Merge branch 'jk/implicit-true' into maint-2.43

Merge branch 'jk/implicit-true' into maint-2.43

Some codepaths did not correctly parse configuration variables
specified with valueless "true", which has been corrected.

* jk/implicit-true:
  fsck: handle NULL value when parsing message config
  trailer: handle NULL value when parsing trailer-specific config
  submodule: handle NULL value when parsing submodule.*.branch
  help: handle NULL value for alias.* config
  trace2: handle NULL values in tr2_sysenv config callback
  setup: handle NULL value when parsing extensions
  config: handle NULL value when parsing non-bools