osv: add omitempty to published and modified fields
osv: add omitempty to published and modified fields

The modified field is required (published is not), but it's useful
for us to be able to store partial OSV entries without a modification
time. Set the omitempty json tag on these fields so they don't
show up with the zero Time.

Change-Id: Id044c84f67ad89bec29ed9f0d1cec1fd3c53105c
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/431435
Reviewed-by: Tatiana Bradley <ed5904c3174de8861076818d9fdd7f7c949a16e6@golang.org>
Run-TryBot: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/vuln,golang/vuln