Field Calculator Combining Fields

Good day all,

I've got a question about the field calculator. I would like to combine several fields together in one, new field. (Street name, House number, Extension, Postal Code and Place). So my code looks like this (the fieldsname are in Dutch)

After a couple of minutes, I get the result in my newly created field. However, not every streetname has a extension (Huisletter) and is left 'blank' in its original field, which results in an extra space in my newly created field.
How do I modify my code to include the number, but exclude the 'blank' cells?

Thanks in advance.