fix(iaas): safely check resp.NetworkInterface for nil

[cgoetz-inovex]

NullableString.IsSet() returns true even if the value is nil:

func (v *NullableString) UnmarshalJSON(src []byte) error {
	v.isSet = true
	return json.Unmarshal(src, &v.value)
}

Description

relates to #1234

Checklist

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

[github-actions]

Merging this branch will not change overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/stackitcloud/stackit-cli/internal/pkg/services/iaas/utils	85.45% (ø)

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/stackitcloud/stackit-cli/internal/pkg/services/iaas/utils/utils.go	85.45% (ø)	110	94	16

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/stackitcloud/stackit-cli/internal/pkg/services/iaas/utils/utils_test.go