Add a null-check to fix a regression in 4de92b7.
Add a null-check to fix a regression in 4de92b7.

The regression (NRE) occurs if null is passed as the argument to an
attribute constructor taking a type:

    [SomeAttribute (null)]

jbevain/cecil,SiliconStudio/Mono.Cecil,cgourlay/cecil,mono/cecil,joj/cecil,fnajera-rac-de/cecil,xen2/cecil,sailro/cecil,kzu/cecil,furesoft/cecil,gluck/cecil,ttRevan/cecil,saynomoo/cecil