Fix reroute ID 0 treated as invalid (#5723)

## Summary

Fixes old logic bug from refactor
https://github.com/Comfy-Org/litegraph.js/pull/602/files

## Changes

- Fixes truthy refactor to explicitly check undefined

## Review Focus

No expectation that this will impact prod, however it may impact
extensions IF someone has explicitly been setting link parentId to 0.
This would be very strange, as it would cause unexpected behaviour in
other parts of the code (which all explicitly check `undefined`).

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5723-Fix-reroute-ID-0-treated-as-invalid-2766d73d365081568124ce1f85cdf84e)
by [Unito](https://www.unito.io)

src/lib/litegraph/src/LLink.ts

@@ -205,7 +205,7 @@ export class LLink implements LinkSegment, Serialisable<SerialisableLLink> {
     network: Pick<ReadonlyLinkNetwork, 'reroutes'>,
     linkSegment: LinkSegment
   ): Reroute[] {
-    if (!linkSegment.parentId) return []
+    if (linkSegment.parentId === undefined) return []
     return network.reroutes.get(linkSegment.parentId)?.getReroutes() ?? []
   }
 
@@ -229,7 +229,7 @@ export class LLink implements LinkSegment, Serialisable<SerialisableLLink> {
     linkSegment: LinkSegment,
     rerouteId: RerouteId
   ): Reroute | null | undefined {
-    if (!linkSegment.parentId) return
+    if (linkSegment.parentId === undefined) return
     return network.reroutes
       .get(linkSegment.parentId)
       ?.findNextReroute(rerouteId)
@@ -498,7 +498,7 @@ export class LLink implements LinkSegment, Serialisable<SerialisableLLink> {
       target_slot: this.target_slot,
       type: this.type
     }
-    if (this.parentId) copy.parentId = this.parentId
+    if (this.parentId !== undefined) copy.parentId = this.parentId
     return copy
   }
 }