fix warning The left operand of '!=' is a garbage value

The 'res' is  uninitialized if params.state == Parameters::PREVIEW.
This sets the default value to 'OK' to silence this warning.

Bug: None
Test: Run clang-tidy on this code and the warning is gone.
Change-Id: I9d1b5e09c7957813f3c66d052c6a9f7d4b074b84