MM-37575: Handle nil value in override_username (#18057)

* MM-37575: Handle nil value in override_username

Since props is a map[string]interface{}, an ", ok" check doesn't
really help because even if a key is not present, by default, a map
will always return the default value of the value, which would be a
nil interface.

So we can just check for the value of the map instead.

https://mattermost.atlassian.net/browse/MM-37575

```release-note
NONE
```

* Fix for integer case as well

```release-note
NONE
```

* Update app/notification.go

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

app/notification.go

@@ -1068,8 +1068,10 @@ func (n *PostNotification) GetSenderName(userNameFormat string, overridesAllowed
 	}
 
 	if overridesAllowed && n.Channel.Type != model.ChannelTypeDirect {
-		if value, ok := n.Post.GetProps()["override_username"]; ok && n.Post.GetProp("from_webhook") == "true" {
+		if value := n.Post.GetProps()["override_username"]; value != nil && n.Post.GetProp("from_webhook") == "true" {
-			return value.(string)
+			if s, ok := value.(string); ok {
+				return s
+			}
 		}
 	}
 

app/notification_test.go

@@ -1799,6 +1799,20 @@ func TestPostNotificationGetSenderName(t *testing.T) {
 		},
 	}
 
+	overriddenPost2 := &model.Post{
+		Props: model.StringInterface{
+			"override_username": nil,
+			"from_webhook":      "true",
+		},
+	}
+
+	overriddenPost3 := &model.Post{
+		Props: model.StringInterface{
+			"override_username": 10,
+			"from_webhook":      "true",
+		},
+	}
+
 	for name, testCase := range map[string]struct {
 		channel        *model.Channel
 		post           *model.Post
@@ -1841,6 +1855,16 @@ func TestPostNotificationGetSenderName(t *testing.T) {
 			allowOverrides: false,
 			expected:       "@" + sender.Username,
 		},
+		"nil override_username": {
+			post:           overriddenPost2,
+			allowOverrides: true,
+			expected:       "@" + sender.Username,
+		},
+		"integer override_username": {
+			post:           overriddenPost3,
+			allowOverrides: true,
+			expected:       "@" + sender.Username,
+		},
 	} {
 		t.Run(name, func(t *testing.T) {
 			channel := defaultChannel