Or, for something more robust:
=IF(COUNT(A2),IF(ISNUMBER(B2),B2,TODAY())-A2,"")

Same result, a few keystrokes shorter:

=IF(COUNT(A2),IF(COUNT(B2),B2,TODAY())-A2,"")

If we make the assumption that if a date exists in B2, that it will always
be an earlier date than today, then we can save a few more keystrokes...

=IF(COUNT(A2),MIN(B2,TODAY())-A2,"")

or, if not always earlier, then if we assume it is always after today,
then...

=IF(COUNT(A2),MAX(B2,TODAY())-A2,"")

Rick