“non well formed numeric value encountered” upon upgrading to 3.10 and php7.3

**What is my ENV:**

`VM server on Hetzner `
`Matomo version: 3.10.0`
`MySQL version: 5.7.26`
`PHP version: 7.3.6-1+0~20190531112735.39+stretch~1.gbp6131b7`

**What is a problem**

Sometimes i get those warning

```
WARNING: /var/www/matomo/vendor/davaxi/sparkline/src/Sparkline/FormatTrait.php(193): Notice - A non well formed numeric value encountered```

