Add edge case handling to ExecutableNodeDTO

Comfy-Org/litegraph.js#1102 addresses most of the edge cases (in particular, KJ get/set nodes) where the new execution flattening for subgraphs breaks existing extensions.

However, the original design of this was targeted specifically for subgraphs and does not address all edge cases. The logic here should be extended to account for situations where:

- Node does not have a normal input for the requested link slot
- `getInputLink` (or override) returns a falsy value

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