Concatenate and referencing a blank cell

I have four columns, and they are "studentnumber", "firstname", "middlename" and "lastname" respectively. I am trying to combine the first initial, the middle initial, and 6 characters of the last name into one string of characters. If there is no middle name, I am trying to put the student number in to replace the middle initial. I have gotten the part where I can combine the characters, however I am having problems with substituting the student number for the middle initial if the middlename field is blank. The incorrect formula I have so far is:

CONCATENATE(LEFT(B2, 1), IF(middlename "", studentnumber, LEFT(C2, 1)) , LEFT(D2, 6))

I don't know how to reference a blank text cell, and I don't know if simply placing the value of the student number field is sufficient to get the value in there. So, any ideas as to how I need to do this?

Thanks FDibbins, yours worked for me!