Fix broken spacing in Form caused by nullable children

[suevalov]

Purpose of PR

• Filter out nullable children in Form component, so the following code doesn't cause weird empty spacing:

<Form>
   {null}
  <TextField />
  <TextField />
</Form>

• Fixed warning about incorrect usage of getDerivedStateFromProps

PR Checklist

• I have read the relevant readme.md file(s)
• All commits follow our Git commit message convention
• Tests are added/updated/not required
• Tests are passing
• Storybook stories are added/updated/not required
• Usage notes are added/updated/not required
• Has been tested based on Contentful's browser support
• Doesn't contain any sensitive information

[wichniowski]

👍