PLT-7077: ignore null array items in slack attachments (#6904)

* ignore null array items in incoming webhooks / command responses

* consolidate code, process announcements in command response as well

* make a bit more idiomatic, add tests

* add missing file

model/slack_attachment_test.go

@@ -0,0 +1,38 @@
+package model
+
+import (
+	"testing"
+)
+
+func TestExpandAnnouncement(t *testing.T) {
+	if ExpandAnnouncement("<!channel> foo <!channel>") != "@channel foo @channel" {
+		t.Fail()
+	}
+}
+
+func TestSlackAnnouncementProcess(t *testing.T) {
+	attachments := SlackAttachments{
+		{
+			Pretext: "<!channel> pretext",
+			Text:    "<!channel> text",
+			Title:   "<!channel> title",
+			Fields: []*SlackAttachmentField{
+				{
+					Title: "foo",
+					Value: "<!channel> bar",
+					Short: true,
+				}, nil,
+			},
+		}, nil,
+	}
+	attachments.Process()
+	if len(attachments) != 1 || len(attachments[0].Fields) != 1 {
+		t.Fail()
+	}
+	if attachments[0].Pretext != "@channel pretext" ||
+		attachments[0].Text != "@channel text" ||
+		attachments[0].Title != "@channel title" ||
+		attachments[0].Fields[0].Value != "@channel bar" {
+		t.Fail()
+	}
+}