Initialize viewport state before missing lookup handling

[sylvesterkaczmarek]

Description

Fix the missing-viewport path in IsaacGetViewportRenderProduct.

OgnIsaacGetViewportRenderProductInternalState.__init__() currently assigns viewport = None to a local variable instead of self.viewport. If get_viewport_from_window_name() cannot resolve the requested viewport on the first compute, the node then evaluates db.per_instance_state.viewport is None before that attribute exists and can raise AttributeError instead of following its documented warning/False path.

Initialize the instance cache correctly with self.viewport = None. Also make the existing warning interpolate the requested viewport name instead of logging the literal {db.inputs.viewport} text.

Validation

• focused regression test forces a viewport lookup miss on a fresh internal state
• verifies compute() returns False rather than raising
• verifies the cache remains None and the node is marked uninitialized
• verifies the warning includes the requested viewport name
• production diff is two corrected lines