Fixed RenderManShader.acceptsInput() crash

RenderManShader.acceptsInput used to crash when you called acceptsInput( None ) on one of its input plugs. This was bad. Now it doesn't. This is good.
added some checks for null plugs in Box's plug promotion methods
Merge pull request #647 from davidsminor/renderManShaderAcceptsInputFix

Fixed some null plug crashes