fix(ske): nil pointer when gateway api is not set

[Manuelvaas]

Description

fixes:

• Provider Crash: SKE resource crashes when gatewayApi is not defined #1671 (nil pointer in SKE)
• "unknown value after apply" for gateway_api => should not be computed
• state drift with applicationLoadBalancer Extension

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[marceljk]

Need to be tested first, if the fix works also with projects where the private preview feature is not available.