Concatenate Multiple variables to a String in Java If Else Block

I need to create a string which becomes an input to a SHA 256 function to generate its hash equivalent. The string is created by concatenating several variables as below:

String strRequest = "";

            strRequest = request_passphrase.concat("access_code=").concat(access_code).concat("amount=").concat(amount).concat("command=").concat(mode).concat("currency=").concat(currency)
            .concat("merchant_identifier=").concat(merchant_identifier)
            .concat(request_passphrase);
        if(strRequest!="" || !strRequest.isEmpty())
        {
        signature = sha256(strRequest);
        }

What should be the best way to create an if-else to drop concatenation for a variable which is null. For ex. if access_code is Null or empty, then the string will be request_passphrase.concat("amount=").concat(amount). and so on.