Allow map name to be empty in DumpNativeStack.

For the gdb jit interface, functions in the stack trace will come from
maps that have no names. Print a special name for these maps and also do
not allow addr2line to run on these maps either.

Test: Build and ran a stack through that use the jit gdb interface.
Change-Id: I39d3f7e24a38df0a02214d1014a5d0b2e0174dd7