How to just gain root permission without running anything?

Use sudo -v:

-v, --validate
Update the user's cached credentials, authenticating the user if necessary.

While muru's answer does exactly what you want, there's also a more general way to "do nothing," even when sudo is not involved.

sudo true

will run the true command, which always succeeds, and has no side effects, like printing to the screen, etc, with sudo. There's also the false command, which always fails. These are both useful in shell scripting.