Update the Begin() of LargeObjectMapSpace

Root cause:
  LargeObjectMapSpace's begin_ is initialized by nullptr.
  so, it's always 0 after std::min

Solution:
  Consider the nullptr case when update begin_

Notes:
  Remove the nullptr check for end_ since it's not needed

Test: make test-art-host-gtest-large_object_space_test

Change-Id: I9918c30916e5d359f0ab23ec2bdb0fc9f093a85f