conf: fix unexpected behavior of config inheritance (#5007)

conf/app.ini

@@ -330,13 +330,13 @@ LEVEL = Trace
 
 ; For "console" mode only
 [log.console]
-; leave empty to inherit
-LEVEL =
+; Comment out to inherit
+; LEVEL =
 
 ; For "file" mode only
 [log.file]
-; leave empty to inherit
-LEVEL =
+; Comment out to inherit
+; LEVEL =
 ; This enables automated log rotate (switch of following options)
 LOG_ROTATE = true
 ; Segment log daily
@@ -350,14 +350,14 @@ MAX_DAYS = 7
 
 ; For "slack" mode only
 [log.slack]
-; leave empty to inherit
-LEVEL =
+; Comment out to inherit
+; LEVEL =
 ; Webhook URL
 URL =
 
 [log.discord]
-; leave empty to inherit
-LEVEL =
+; Comment out to inherit
+; LEVEL =
 ; Webhook URL
 URL =
 ; Username displayed in webhook