You have added a new TransfromColorSpace method(in version 7.0.0.0015), it works in case if source and target profiles is not null.
How can I transform image to SRBG color profile if it does not contain color profile(source profile is null) ?
With the TransformColorSpace method you will have to specify a source and a target profile. The advantage of this method is that the source profile will not be used it the image already contains a profile. To transfer the image to sRGB you could use ColorProfile.SRGB as the target profile. You will have to pick the source profile yourself.
How can I determine which color profile used for image in that case?
It looks like you already answered your own question: I have a image with ColorProfile.AdobeRGB1998. But you must be asking something else. Can you clarify your question?