I am creating the test case for the method as in attached file. I am using mockito here as my method have external dependencies. I have issue at in my method that i am testing, but not actually find out.
Test code like this is why people rail against Mocks and proclaim that mocks are evil.
This is very smelly test code. What your test code is telling you is: "This design is hard to test!"
Listen to your test code and go back and rethink your design. Why does your design require so much test setup? Are your objects leaking too much implementation detail? Are there way too many collaborations? Do some collaborations need to be pushed down further away, so higher-level test code doesn't have to care about them?
Solving your problem with the null return value is just delaying the inevitable pain you will suffer later on if you continue with your current design. And I can most certainly assure you that the pain you will feel later will be much greater than the pain you feel now. Much, much greater.