[Form] add missing whitespace control modifier in Twig form template

| Q             | A
| ------------- | ---
| Bug fix?      | kind of (after updating from 2.3.17 to 2.3.18, some of my tests were broken because of this)
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | they should, let's see what Travis has to say...
| Fixed tickets | --
| License       | MIT
| Doc PR        | --

This adds a whitespace control modifier which was missing in #11386.

after merging #11386:
```html
<input type="hidden" id="myfield" name="myfield"     value="blah" />
```

before merging #11386 and with this PR again:
```html
<input type="hidden" id="myfield" name="myfield" value="blah" />
```