Fix #147860: Default input setting for node declarations is unintialized

This has ripple effects by making the "hide value" setting ineffective and
always hiding socket values as well as graying out the "hide value" node group
option.

Pull Request: https://projects.blender.org/blender/blender/pulls/148188

source/blender/nodes/NOD_node_declaration.hh

@@ -256,7 +256,7 @@ class SocketDeclaration : public ItemDeclaration {
 
  public:
   /** Some input sockets can have non-trivial values in the case when they are unlinked. */
-  NodeDefaultInputType default_input_type;
+  NodeDefaultInputType default_input_type = NodeDefaultInputType::NODE_DEFAULT_INPUT_VALUE;
   /**
    * Property that stores the name of the socket so that it can be modified directly from the
    * node without going to the side-bar.