Fix exceptions raised when accessing attributes that either do not exist or are not associated to a graph

[cbentejac]

Description

This PR fixes two exceptions related to attributes that were raised in some cases:

• In the Viewer2D, when determining whether the active node relates to the lens distortion viewer, check if the "input" attribute exists for that node before trying to access it. (fixes a KeyError exception)
• When the attribute's full name and full label within the graph are evaluated, check that the node it belongs to is associated to an existing graph (some nodes may be created temporarily in some specific cases without ever being attached to a graph): if it is, then the attribute's full name/label can be returned, otherwise "UNDEFINED" is. (fixes a ValueError exception)