Fix env config parsing for "GITEA____APP_NAME" (#26001) (#26013)

Backport #26001 by @wxiaoguang

Regression of #24832 

Fix the bug and add a test for it

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/setting/config_env.go

@@ -86,7 +86,7 @@ func decodeEnvSectionKey(encoded string) (ok bool, section, key string) {
 		key += remaining
 	}
 	section = strings.ToLower(section)
-	ok = section != "" && key != ""
+	ok = key != ""
 	if !ok {
 		section = ""
 		key = ""