NULL values in field(s) or row(s) handling in SSIS Expression

Use the below construct to convert your field from empty string to NULL.

ColumnName == "" ? NULL(DT_WSTR,50) : ColumnName

I have used the above construct in my Derived Column Transformation editor as follows:

To eliminate NULL rows we can do a check of any particular column to eliminate the row as below in Conditional Split:

ISNULL(FUND_ID) ==  FALSE