Update BSONStorage.java

Based on the code, 
writeField#186 will pass the `toIgnore` as null, which will then be used in getTypeForBSON#154.
If i need to get into that `if` block on #154, (with respect to the comments between 149-153) i just cannot, since fs[0].getName will have to be null for it to equal `toIgnore`. and if its null, it will just throw an NPE.

All i want to do is create an array ["a", "b"] and that doesn't seem possible with the current code