Handing missing Disabled field in OIDC extension data to avoid crash

[koala7659]

Handling exception case for Patch operation when Disabled field is missing from OIDC extension data in the shoot like following:

  extensions:
    - type: shoot-oidc-service
    - type: shoot-dns-service

In the old implementation the missing Disabled field was causing unexpected crash.

In the new implementation the missing Disabled filed is interpreted as "Enabled by default"