fix: only copy attributes if they exist

[vincentullmann]

Changelog Description

Only copy fps and source if they exists on version_attributes

Additional review information

Its not uncommon for us to ingest nukenodes via the traypublisher (eg.: as part of deliveries from third party vendors)
Those products do not have an fps attribute and don't really need one either.

During load error happened on Product: "nukenodesLensDistortion" Representation: "nk" Version: 1

Error message: 'fps'

Traceback (most recent call last):
  File "/home/v/dev/ayon-core/client/ayon_core/tools/loader/models/actions.py", line 1005, in _load_representations_by_loader
    load_with_repre_context(
  File "/home/v/dev/ayon-core/client/ayon_core/pipeline/load/utils.py", line 327, in load_with_repre_context
    return loader.load(repre_context, name, namespace, options)
  File "/home/v/dev/ayon-core/client/ayon_core/pipeline/load/plugins.py", line 430, in wrapped_method
    result = original_method(self, *args, **kwargs)
  File "/home/v/dev/ayon-nuke/client/ayon_nuke/plugins/load/load_backdrop.py", line 70, in load
    data_imprint[k] = version_attributes[k]
KeyError: 'fps'

Testing notes:

1. in nuke: create a node and export it into a file (nuke.nodeCopy("/mnt/d/work/nodes.nk")
2. configure trayPublisher to publish a nukenodes product type
3. publish the nodes.nk from trayPublisher
4. load them back into nuke via AYON

example tray publisher config:

{
  "simple_creators": [
    {
      "icon": "",
      "label": "Lens Distortion Node",
      "extensions": [
        ".nk"
      ],
      "identifier": "",
      "description": "Ingest Nuke Lens Distortion Nodes from Tracking Outsource",
      "allow_sequences": false,
      "default_variants": [
        "LensDistortion"
      ],
      "product_base_type": "nukenodes",
      "product_type_items": [],
      "allow_multiple_items": false,
      "detailed_description": "",
      "allow_version_control": true
    }
  ]
}

[BigRoy]

Seems like a decent fix. At the same time, I personally don't see why we'd want to imprint this data to begin with. I feel like we're better off at some point removing these from being imprinted at all. @jakubjezek001 thoughts?