One way to handle a blank response to an interval-scaled item with a midpoint would be to assign the midpoint in the scale as the response to that particular item.
Another way is to allow the computer to ignore the blank responses when the analyses are done. This, of course, will reduce the sample size whenever that variable is involved in the analyses.
A third way is to assign to the item the mean value of the responses of all those who have responded to that particular item.
A fourth is to give the item the mean of the responses of this particular respondent to all other questions measuring this variable.
A fifth way of dealing with it is to give the missing response a random number within the range for that scale. It should also be noted that SPSS uses linear interpolation from adjacent points as also a linear trend to replace missing data.