In the MoveConstructor/MoveAssignment tests, a check for a `NULL` allocator
return has been used to check for the correct moved-from state of a Document.
After the merge of #426, the GetAllocator call fails with an assertion,
if the allocator of a GenericDocument is NULL.

Check for the throw, instead of NULL in the move-related tests.
Tested with GCC 4.9 on Linux with C++11 enabled.

Reported-by: @woldendans