Fix ghost error in validation function when group node has issues

[sebastianrath]

This PR fixes a cascading validation error in group nodes where a sub-graph error causes the parent graph to falsely report the group node as non-existent. While the group node may have issues, it should be still considered as existing, otherwise it would lead to a misleading error.

So for example when a core/group@v1 node has some errors, the group factory returned nil instead of the group node. This caused the group node to never be registered in the parent graphs node map, producing a misleading secondary error for any connection referencing the group.

Before:

  $ actrun validate test.act
  ❌ Graph validation failed with 2 error(s):
  --- Error 1 ---
  ❌ unknown node type 'core/does-not-exist@v1'

  --- Error 2 ---
  ❌ connection dst node 'g' does not exist

Error 2 is a false positive, node g is declared in the graph, just has a bug.

Now:

  $ actrun validate test.act
  ❌ Graph validation failed with 1 error(s):
  --- Error 1 ---
  ❌ unknown node type 'core/does-not-exist@v1'

Example graph error:

entry: s
type: generic
nodes:
  - id: s
    type: core/start@v1
  - id: g
    type: core/group@v1
    graph:
      entry: gi
      type: group
      nodes:
        - id: gi
          type: core/group-inputs@v1
        - id: go
          type: core/group-outputs@v1
        - id: bad
          type: core/does-not-exist@v1
      connections: []
      executions: []
      inputs:
        exec-in: { type: "", index: 0, exec: true }
      outputs:
        exec-success: { type: "", index: 0, exec: true }
connections: []
executions:
  - src: { node: s, port: exec }
    dst: { node: g, port: exec-in }