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 <tatiana@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
diff --git a/osv/json.go b/osv/json.go
index 3160fda..d3bc865 100644
--- a/osv/json.go
+++ b/osv/json.go
@@ -182,8 +182,8 @@
 // entry
 type Entry struct {
 	ID         string      `json:"id"`
-	Published  time.Time   `json:"published"`
-	Modified   time.Time   `json:"modified"`
+	Published  time.Time   `json:"published,omitempty"`
+	Modified   time.Time   `json:"modified,omitempty"`
 	Withdrawn  *time.Time  `json:"withdrawn,omitempty"`
 	Aliases    []string    `json:"aliases,omitempty"`
 	Details    string      `json:"details"`