[FormHelperText] Add a min-height

[oliviertassinari]

Add a min-height as the flexbox layout with wrap mode might not be enough.
One might want to do the following the get his text-fields with a consistent height even when one field has an helper text:

<TextField
  id="name"
  label="Name"
  value={this.state.name}
  onChange={event => this.setState({ name: event.target.value })}
  helperText="helperText"
/>
<TextField
  id="uncontrolled"
  label="Uncontrolled"
  defaultValue="foo"
  className={classes.input}
  helperText={<span />}
/>

Closes #7068

• PR has tests / docs demo, and is linted.
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s) (http://tr.im/vFqem).