This used to be a major problem with me, too, until I discovered the Nz function. Its syntax is Nz(TestValue,ValueIfNull).
If the TestValue is Null, then the ValueIfNull is returned. Otherwise the TestValue is returned. So
Nz(Null,"N/A") returns N/A
Nz(Null,0) returns 0
Nz(10,0) returns 10...