[Form] Do not render default ID attribute on empty form name
[Form] Do not render default ID attribute on empty form name

Bug fix: no
Feature addition: no
Backwards compatibility break: no
Symfony2 tests pass: yes
Fixes the following tickets: -
Todo: -

This is cleanup after enabling empty form names, now form with empty name
will not render the default `id="form"` container attribute.
Developers can extend/override this behaviour by standard form theming methods.

symfony/twig-bridge,symfony/TwigBridge