[lldb] Re-add nullptr check to IRForTarget::RewriteObjCConstString log statement

The nullptr check here was removed in 4ef50a33
when I replaced (nearly) all log->Print to LLDB_LOG calls (which automatically
check for this stuff). But it seems this one call escaped my sed call.

Currently working on a test that can cover this code path but we can revert
this until I have found one.