MM-13492 Allow empty channel in /channels/view (#10062)

* Allow empty channel in /channels/view

* Simplify if statement.

api4/channel.go

@@ -917,11 +917,22 @@ func viewChannel(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 
 	view := model.ChannelViewFromJson(r.Body)
-	if view == nil || !model.IsValidId(view.ChannelId) || (view.PrevChannelId != "" && !model.IsValidId(view.PrevChannelId)) {
+	if view == nil {
 		c.SetInvalidParam("channel_view")
 		return
 	}
 
+	// Validate view struct
+	// Check IDs are valid or blank. Blank IDs are used to denote focus loss or inital channel view.
+	if view.ChannelId != "" && !model.IsValidId(view.ChannelId) {
+		c.SetInvalidParam("channel_view.channel_id")
+		return
+	}
+	if view.PrevChannelId != "" && !model.IsValidId(view.PrevChannelId) {
+		c.SetInvalidParam("channel_view.prev_channel_id")
+		return
+	}
+
 	times, err := c.App.ViewChannel(view, c.Params.UserId, !c.App.Session.IsMobileApp())
 	if err != nil {
 		c.Err = err

api4/channel_test.go

@@ -1418,6 +1418,12 @@ func TestViewChannel(t *testing.T) {
 	_, resp = Client.ViewChannel(th.BasicUser.Id, view)
 	CheckBadRequestStatus(t, resp)
 
+	// All blank is OK we use it for clicking off of the browser.
+	view.PrevChannelId = ""
+	view.ChannelId = ""
+	_, resp = Client.ViewChannel(th.BasicUser.Id, view)
+	CheckNoError(t, resp)
+
 	view.PrevChannelId = ""
 	view.ChannelId = "junk"
 	_, resp = Client.ViewChannel(th.BasicUser.Id, view)