Uploadable YAML Driver problem when using both Uploadable and Timestampable behaviors on an Entity

When using both Uploadable and Timestampable behaviors on an Entity, the Yaml Driver throws an error when trying to access :

```
  [ErrorException]                                                                                                                             
  Notice: Undefined offset: 0 in /path-to-project/vendor/gedmo/doctrine-extensions/lib/Gedmo/Uploadable/Mapping/Driver/Yaml.php line 68
```
The "0" index of this array exists in Uploadable config but not in Timestampable config. That's where it breaks.

My YAML config :

```yml
Acme\FileBundle\Entity\File:
  type: entity
  table: null
  repositoryClass: Acme\FileBundle\Entity\FileRepository
  id:
    id:
      type: integer
      id: true
      generator:
        strategy: AUTO
  gedmo:
    uploadable:
      allowOverwrite: false
      appendNumber: true
      filenameGenerator: ALPHANUMERIC
  fields:
    name:
      type: string
      length: 255
      nullable: true
    path:
      type: string
      gedmo:
        - uploadableFilePath
    mimeType:
      type: string
      gedmo:
        - uploadableFileMimeType
      nullable: true
    size:
      type: decimal
      gedmo:
        - uploadableFileSize
      nullable: true
    clientFirstName:
      type: string
      length: 100
      nullable: true
    clientLastName:
      type: string
      length: 100
      nullable: true
    clientEmail:
      type: string
      length: 100
      nullable: true
    clientCompany:
      type: string
      length: 255
      nullable: true
    username:
      type: string
      length: 100
    userFirstName:
      type: string
      length: 100
      nullable: true
    userLastName:
      type: string
      length: 100
      nullable: true
    userEmail:
      type: string
      length: 255
      nullable: true
    code:
      type: string
      length: 20
    lifetime:
      type: integer
      length: 4
    lifetimeType:
      type: integer
      length: 4
    ipAddress:
      type: string
      length: 15
      nullable: true
    nbDownloads:
      type: integer
      length: 4
      default: 0
    active:
      type: boolean
      default: true
    createdAt:
      type: date
      gedmo:
        timestampable:
          on: create
    updatedAt:
      type: datetime
      gedmo:
        timestampable:
          on: update
```