MM-31062: Rewrite empty string checks to be more idiomatic (#16587)

https://mattermost.atlassian.net/browse/MM-31062

```release-note
NONE
```

store/sqlstore/audit_store.go

@@ -57,7 +57,7 @@ func (s SqlAuditStore) Get(userId string, offset int, limit int) (model.Audits,
 		Limit(uint64(limit)).
 		Offset(uint64(offset))
 
-	if len(userId) != 0 {
+	if userId != "" {
 		query = query.Where(sq.Eq{"UserId": userId})
 	}
 

store/sqlstore/group_store.go

@@ -91,7 +91,7 @@ func (s *SqlGroupStore) createIndexesIfNotExists() {
 }
 
 func (s *SqlGroupStore) Create(group *model.Group) (*model.Group, error) {
-	if len(group.Id) != 0 {
+	if group.Id != "" {
 		return nil, store.NewErrInvalidInput("Group", "id", group.Id)
 	}
 

store/sqlstore/user_store.go

@@ -286,7 +286,7 @@ func (us SqlUserStore) UpdateAuthData(userId string, service string, authData *s
 			     AuthService = :AuthService,
 			     AuthData = :AuthData`
 
-	if len(email) != 0 {
+	if email != "" {
 		query += ", Email = lower(:Email)"
 	}