Noel, Although a 4506-T (request for tax transcript) should never be sent out in blank, the solution is simple, call the lender and tell them that you will not sign the form until it has been properly completed.
Why would any credible lender sent that form blank, beats me, maybe they hired a new employee that didn't follow procedures, or a loan officer got lazy, but the bottom line is, don't sign it until they have completed, Income does not have to be verified on a FHA streamline, so the form is necessary.
What could a lender do with this form in its blank state? They could fill it out and request all items listed in 6 a-6, 7 & 8 for the past 4 years.