Allow NullConstant to be untyped in GraphVisualiser.

The NullConstant may be added to the graph during other passes that
happen between ReferenceTypePropagation and Inliner (e.g.
InstructionSimplifier). If the inliner doesn't run or doesn't inline
anything, the NullConstant remains untyped.

The infrastructure to properly type NullConstants everywhere is to
complex to add for the benefits

Bug: 25786318

Change-Id: I904a3e605b57f8cac9936e82f19a4994c7b1a82a
Signed-off-by: Mark Mendell <mark.p.mendell@intel.com>