fix: always set originalLockfileVersion when doing shrinkwrap reset

Fix: npm/cli#3899

A shrinkwrap reset without specifying a lockfile version was triggering
an old lockfile warning due to the originalLockfileVersion not being set
prior to checking whether the lockfile was old.

lib/shrinkwrap.js

@@ -380,9 +380,10 @@ class Shrinkwrap {
   reset () {
     this.tree = null
     this[_awaitingUpdate] = new Map()
-    this.originalLockfileVersion = this.lockfileVersion
+    const lockfileVersion = this.lockfileVersion || defaultLockfileVersion
+    this.originalLockfileVersion = lockfileVersion
     this.data = {
-      lockfileVersion: this.lockfileVersion || defaultLockfileVersion,
+      lockfileVersion,
       requires: true,
       packages: {},
       dependencies: {},

test/arborist/build-ideal-tree.js

@@ -611,6 +611,33 @@ t.test('force a new mkdirp (but not semver major)', async t => {
   t.equal(arb.idealTree.children.get('minimist').package.version, '1.2.5')
 })
 
+t.test('empty update should not trigger old lockfile', async t => {
+  const path = t.testdir({
+    'package.json': JSON.stringify({
+      name: 'empty-update',
+      version: '1.0.0',
+    }),
+    'package-lock.json': JSON.stringify({
+      name: 'empty-update',
+      version: '1.0.0',
+      lockfileVersion: 2,
+      requires: true,
+      packages: {
+        '': {
+          name: 'empty-update',
+          version: '1.0.0',
+        }
+      }
+    })
+  })
+  const checkLogs = warningTracker()
+
+  const arb = newArb(path)
+  await arb.reify({ update: true })
+
+  t.strictSame(checkLogs(), [])
+})
+
 t.test('no fix available', async t => {
   const path = resolve(fixtures, 'audit-mkdirp/mkdirp-unfixable')
   const checkLogs = warningTracker()