Avoid setting an empty value on slash command IconURL (#36327)

* Avoid setting an empty value on slash command IconURL

When `PostEnablePostIconOverride` is enabled and no icon URL is
provided, the override icon URL was being set to empty and triggering
a warning. This change updates the behavior not to set the icon at
all, avoiding the triggering of the warn message while keeping the
behavior.

* Adds an additional check to the test

---------

Co-authored-by: Miguel de la Cruz <miguel@ctrlz.es>

server/channels/app/command.go

@@ -672,8 +672,6 @@ func (a *App) HandleCommandResponsePost(rctx request.CTX, command *model.Command
 		} else if response.IconURL != "" {
 			post.AddProp(model.PostPropsOverrideIconURL, response.IconURL)
 			isBotPost = true
-		} else {
-			post.AddProp(model.PostPropsOverrideIconURL, "")
 		}
 	}
 

server/channels/app/slashcommands/command_test.go

@@ -250,6 +250,17 @@ func TestHandleCommandResponsePost(t *testing.T) {
 	assert.Equal(t, resp.IconURL, post.GetProp(model.PostPropsOverrideIconURL))
 	assert.Equal(t, "true", post.GetProp(model.PostPropsFromWebhook))
 
+	resp.IconURL = ""
+
+	// When both command and response icon URLs are empty and EnablePostIconOverride is enabled,
+	// override_icon_url must not be set (avoids spurious "prop must be a valid URL" warning).
+	post, err = th.App.HandleCommandResponsePost(th.Context, command, args, resp, builtIn)
+	assert.Nil(t, err)
+	assert.Nil(t, post.GetProp(model.PostPropsOverrideIconURL))
+	assert.NotContains(t, post.GetProps(), model.PostPropsOverrideIconURL)
+
+	resp.IconURL = "Response icon url"
+
 	// Test Slack text conversion.
 	resp.Text = "<!channel>"