fix low level setting - add null check for attribute name

[zaqifathis]

sample test file: Tests/test/org/bimserver/tests/lowlevel/TestCreateLists.java

[hlg]

This might make the test running without failure, but I am not so sure whether this is the behavior we want or whether the ...AsString list should be created in the first place (earlier in the test, during is applying AddAttributeChange). Uploading a file always keeps original strings for doubles in "...AsString" attributes, but the low level methods seem to be a bit inconsistent. Other methods might rely on this as well. Although IFC serializers apparently don't rely on the ...AsString attributes to be set, implementation of issue #765, would require them to be always set, for example.

Also this is an example of a test that does too much, so if something fails, it is not clear what is the failure. Adding assertions about the ..AsString attributes might mitigate this as well.