sdk: fix bug with returning default value from GetOk

[fairclothjm]

This very well may be a breaking change but I wanted to run the full Vault test suite to see what happens.

[github-actions]

CI Results: failed ❌
Failures:

Test Type	Package	Test	Logs
race	builtin/credential/approle	TestAppRole_BoundCIDRLogin	view test results
race	builtin/credential/approle	TestAppRole_RoleCRUD	view test results
race	builtin/credential/approle	TestAppRole_RoleConstraints	view test results
race	builtin/credential/aws	TestAwsEc2_RoleCrud	view test results
race	builtin/credential/aws	TestBackend_pathIam	view test results
race	builtin/credential/aws	TestBackend_pathRoleMixedTypes	view test results
race	builtin/credential/aws	Test_enableIamIDResolution	view test results
race	builtin/credential/ldap	TestLdapAuthBackend_ConfigUpgrade	view test results
race	builtin/logical/aws	TestBackend_PathConfigRoot	view test results
race	builtin/logical/database	TestBackend_StaticRole_Rotation_Schedule_ErrorRecover	view test results

and 101 - 10 other tests

[github-actions]

Build Results:
All builds succeeded! ✅

[fairclothjm]

This is indeed a breaking change!