Nodes: use better default for color sockets

Using a fully transparent black is default never was an explicit decision.
Now the default is a light grey, that we use in many nodes already.

source/blender/nodes/NOD_socket_declarations.hh

@@ -120,7 +120,7 @@ class ColorBuilder;
 
 class Color : public SocketDeclaration {
  public:
-  ColorGeometry4f default_value;
+  ColorGeometry4f default_value{0.8f, 0.8f, 0.8f, 1.0f};
 
   friend ColorBuilder;