[DomCrawler] Textarea value should default to empty string instead of null.

Noticed this when working on Updating Behat/Mink/Goutte to Guzzle 4, see https://github.com/fabpot/Goutte/pull/140 and https://github.com/Behat/MinkGoutteDriver/pull/34. 

Testing this with Drupal 8, trying to submit a Form which has empty form elements, the $value in a TextArea form elements defaults to null, then we end up with those in getValues() and then http_build_query() creates an empty string for those. That results in array keys like ['' => false]. This is then passed through to Guzzle, which ends up in MessageFactory::addPostData(), goes into the else creates and tries to create file with those values, which will throw an exception in GuzzleHttp\Stream::create(): with "Invalid resource type false".

I think the correct fix is to default to an empty string instead of null.


Will also need tests, 

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        |