Derived Column Logic To Replace Empty String With Null?

Ok.. so I have a fixed position data feed. I read the file in as just whole rows initially, process a specific position and evaluate a conditional split to determine direction of the file for proper processing (file contains multiple recors with different layouts). This all works fine. I then use the derived column feature to process all the columns.

Most of the columns are as simple as SUBSTRING(RecordData,1,10) for example to get the first column. This all works for string data. I then have a column that is a date field. The problem occurs that the code SUBSTRING(RecordData,20,10) returns either a date or empty set of data if no date was in the original file. When this gets sent to the OLEDB connection (SQL Server 2005) into the date field it fails. If the record has a date it works, but if it is empty it fails the insert.

I tried to replace empty strings with NULLs with this code. REPLACE(TRIM(SUBSTRING(RecordData,20,10)),"",NULL(DT_WSTR,10)). This does not work. So my question is how do I bring a date field from a fixed flat file into a SQL datetime field using a derived column? More specifically is how do I set it to NULL if its empty data? When I use the above code it inserts all the rows from the file, but it sets all rows to NULL not just the empty ones.

Thanks.