Prevent phantom dates when combining dates with potentially blank cells?

I'm trying to combine four cells with dates into one, with + as a delimiter. Some of the cells are empty. I tried the TEXTJOIN function, but Excel returns a date for the empty cells.

For example given this data, where cell 4 is empty:

My function is

=TEXTJOIN("+";TRUE;TEXT(N2;"dd.mm.yyyy");TEXT(U2;"dd.mm.yyyy");TEXT(AB2; "dd.mm.yyyy");TEXT(AI2;"dd.mm.yyyy"))

the formula gives me the following:

23.8.2016+08.06.2016+29.9.1999+00.01.1900

Is there a way around this - or another formula I can use?