The pattern with Optional is for one to avoid returningnull. It's still perfectly possible to pass in null to a method.
While these aren't really official yet, you can use JSR-308 style annotations to indicate whether or not you accept null values into the function. Note that you'd have to have the right tooling to actually identify it, and it'd provide more of a static check than an enforceable runtime policy, but it would help.
public int calculateSomething(@NotNull final String p1, @NotNull final String p2) {}