fix(dependencies-hierarchy): initialize resultHeight to null

In a debugger, I noticed a test package with no dependencies return a
height of `0` from getTree. By the comment in `DependencyInfo`, this
should instead be `null` since the dependencies array is empty.

It turns out this is the case when `deps == null`. The `getTree`
function early returns with a `null` height:

```ts
if (deps == null) {
  return { dependencies: [], isPartiallyVisited: false, height: null }
}
```

However, when `deps` is `{}`, `resultHeight` gets initialized to `0`.
The subsequent `.forEach` loop then iterates over the non-empty object.

reviewing/dependencies-hierarchy/src/getTree.ts

@@ -80,7 +80,7 @@ function getTreeHelper (
   const peers = new Set(Object.keys(opts.currentPackages[parentId].peerDependencies ?? {}))
 
   const resultDependencies: PackageNode[] = []
-  let resultHeight = 0
+  let resultHeight: number | null = null
   let resultCircular: boolean = false
   let resultIsPartiallyVisited = false
 
@@ -119,7 +119,7 @@ function getTreeHelper (
           const children = getChildrenTree(keypath.concat([relativeId]), relativeId)
           dependencies = children.dependencies
           const heightOfCurrentDepNode = children.height == null ? 0 : children.height + 1
-          resultHeight = Math.max(resultHeight, heightOfCurrentDepNode + 1)
+          resultHeight = Math.max(resultHeight ?? 0, heightOfCurrentDepNode + 1)
           resultIsPartiallyVisited = resultIsPartiallyVisited || children.isPartiallyVisited
 
           if (children.circular) {