modfile: handle indirect dependencies with an empty comment

This was reported in golang/go#35737.

Change-Id: I67383fbf668adea1c22feb7f8d25cd1652edcea5
Reviewed-on: https://go-review.googlesource.com/c/mod/+/208273
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
2 files changed