The solution that I ended up using... Since the Transport field was s string field instead of using IsNull in an If statement I used the LEN function.
So essentially if the field was not null then the len of the transport field would be greater than 0.
So with some modification to my LEFT JOIN paragraph I ended up with the following to solve my issue and populate the table correctly
LEFT JOIN (TRANSPORT_DATA_INT)
RESIDENT TRANSPORT_DATA_INT GROUP BY [Rev-Trac#];