fix(arborist): safeguard against null node.target in flag calculation (#7579)

<!-- What / Why -->
If a node represents a symbolic link or a file dep (node.isLink is
true), its target is expected to reference another node in the
dependency tree. If the linking is not done correctly or is incomplete,
node.target might be null.
<!-- Describe the request in detail. What it does and why it's being
changed. -->
in this PR, a null check is added to ensure node.target is not null or
before proceeding, which will prevent causing errors like:
`npm error Cannot set properties of null (setting 'peer')` 

## References
  Related to #7065, 
  Fixes #6622, #5007,
  Closes #6622, #5007

Author: AmirSa12

workspaces/arborist/lib/calc-dep-flags.js

@@ -31,6 +31,10 @@ const calcDepFlagsStep = (node) => {
 
   // for links, map their hierarchy appropriately
   if (node.isLink) {
+    // node.target can be null, we check to ensure it's not null before proceeding
+    if (node.target == null) {
+      return node
+    }
     node.target.dev = node.dev
     node.target.optional = node.optional
     node.target.devOptional = node.devOptional
@@ -97,15 +101,18 @@ const unsetFlag = (node, flag) => {
       tree: node,
       visit: node => {
         node.extraneous = node[flag] = false
-        if (node.isLink) {
+        if (node.isLink && node.target) {
           node.target.extraneous = node.target[flag] = false
         }
       },
       getChildren: node => {
         const children = []
-        for (const edge of node.target.edgesOut.values()) {
-          if (edge.to && edge.to[flag] &&
-            (flag !== 'peer' && edge.type === 'peer' || edge.type === 'prod')
+        const targetNode = node.isLink && node.target ? node.target : node
+        for (const edge of targetNode.edgesOut.values()) {
+          if (
+            edge.to &&
+            edge.to[flag] &&
+            ((flag !== 'peer' && edge.type === 'peer') || edge.type === 'prod')
           ) {
             children.push(edge.to)
           }

workspaces/arborist/test/calc-dep-flags.js

@@ -264,3 +264,16 @@ t.test('set parents to not extraneous when visiting', t => {
   t.equal(bazLink.devOptional, false, 'bazlink not devOptional')
   t.end()
 })
+
+t.test('check null target in link', async t => {
+  const root = new Link({
+    path: '/some/path',
+    realpath: '/some/path',
+    pkg: {
+      dependencies: { foo: '' },
+    },
+  })
+  t.doesNotThrow(() => calcDepFlags(root))
+  t.doesNotThrow(() => calcDepFlags(root, false))
+  t.end()
+})