grub2-mkconfig が "no command and will do nothing" エラーで失敗する理由

Issue

• grub2-mkconfig grub 設定ファイルの生成に失敗しました。

[root@rhel8 ~]# grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg
Generating grub configuration file ...
Script `/boot/efi/EFI/redhat/grub.cfg.new' contains no commands and will do nothing
Syntax errors are detected in generated GRUB config file.
Ensure that there are no errors in /etc/default/grub
and /etc/grub.d/* files or please file a bug report with
/boot/efi/EFI/redhat/grub.cfg.new file attached.

作成された /boot/efi/EFI/redhat/grub.cfg.new は空のファイルです。

[root@rhel8 ~]# cat /boot/efi/EFI/redhat/grub.cfg.new
#
# DO NOT EDIT THIS FILE
#
# It is automatically generated by grub2-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#