Skip DisableNodeCaching instance conversion on subsequent attempts

[kiersten-stokes]

Fixes #2979

What changes were proposed in this pull request?

For pipeline export and submit, a PipelineDefinition object is initialized multiple times. Any property that is converted to an ElyraProperty, however, only needs to be converted once. An attempt to convert what is already an ElyraProperty will result in unexpected behavior, as seen in #2979. This case is already covered for all properties besides DisableNodeCaching here.

Because DisableNodeCaching currently requires special handling (until #2969 is closed), the same if statement linked previously is also added to it's one-off get_single_instance method.

How was this pull request tested?

Tested manually, confirming that pipelines.kubeflow.org/max_cache_staleness: P0D appears in the YAML.

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[ptitzler]

LGTM!