chore(node): Make the PackageInfo.Children.manifest nullable

Yarn Berry outputs a `null` value the the `manifest` for virtual
dependencies, to prepare for an upcoming change.

Signed-off-by: Frank Viernau <[email protected]>

Author: fviernau

plugins/package-managers/node/src/main/kotlin/yarn2/PackageInfo.kt

@@ -38,8 +38,9 @@ internal data class PackageInfo(
     data class Children(
         @SerialName("Version")
         val version: String,
+        /** Can be null for virtual dependencies */
         @SerialName("Manifest")
-        val manifest: Manifest,
+        val manifest: Manifest? = null,
         @SerialName("Dependencies")
         val dependencies: List<Dependency> = emptyList()
     )