[Form] Fixed "label" option to accept the value "0"

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6862
| License       | MIT
| Doc PR        | -

@fabpot: This commit will cause troubles when merging 2.0 into 2.1 and up. Tell me if you need help upon conflict resolution.