[FrameworkBundle][Form] Enable empty value if choice field not required

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | possible
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

When the field is not required, the empty value should be selectable.