Fix error message in default version fact

A commit that I merged yesterday broke the default version fact
such that it would sometimes return nil and sometimes an error message
if your distro wasn't supported.  This commit makes it consistent again.

lib/facter/postgres_default_version.rb

@@ -48,15 +48,27 @@ end
 
 Facter.add("postgres_default_version") do
   setcode do
-    case Facter.value('osfamily')
-      when 'RedHat'
-        get_redhatfamily_postgres_version()
-      when 'Linux'
-        get_redhatfamily_postgres_version()
-      when 'Debian'
-        get_debianfamily_postgres_version()
-      else
-        "Unsupported OS!  Please check `postgres_default_version` fact."
+    result =
+      case Facter.value('osfamily')
+        when 'RedHat'
+          get_redhatfamily_postgres_version()
+        when 'Linux'
+          get_redhatfamily_postgres_version()
+        when 'Debian'
+          get_debianfamily_postgres_version()
+        else
+          nil
+      end
+
+    # TODO: not sure if this is really a great idea, but elsewhere in the code
+    # it is useful to be able to distinguish between the case where the fact
+    # does not exist at all (e.g., if pluginsync is not enabled), and the case
+    # where the fact is not known for the OS in question.  It might be better
+    # to use a shorter sentinel value here and then check for it elsewhere,
+    # e.g. in the same place we're checking for nil and warning about pluginsync.
+    if result == nil
+      result = "Unsupported OS!  Please check `postgres_default_version` fact."
     end
+    result
   end
-end
+end