src/go/doc: in Examples, check for len of params.List, not nilness

src/go/doc: in Examples, check for len of params.List, not nilness

This makes the check the same as the one in the tests vet check.
It's safer to check the number of arguments rather than for a nil
slice.

Change-Id: I8e04e9c612573f334770c1c4245238649656c6e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/211598
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>