Fix visibility of the Install button in packages grid (#16675)

### What does it do?
Adds a new prop to provide a boolean indicator of a package's installed
state. The prop utilizes the recently-introduced date formatter's
`isEmpty` method, which more comprehensively determines whether the
`installed` field contains a meaningful value.

Note that this new prop still relies on a derived value. It would be
much better IMO at some future point to persist this state in a new
database field rather than basing it off of a date value.

### Why is it needed?
Install button is currently missing in the grid.

### How to test
Download, install, and uninstall a few packages to verify the install
button appears as expected.

### Related issue(s)/PR(s)
Resolves #16672

core/src/Revolution/Processors/Workspace/Packages/GetList.php

@@ -108,14 +108,14 @@ public function getSortClassKey()
      */
     public function prepareRow(xPDOObject $object)
     {
-        if ($object->get('installed') === '0000-00-00 00:00:00') {
-            $object->set('installed', null);
-        }
         $packageArray = $object->toArray();
+
+        $isInstalled = !$this->formatter->isEmpty($packageArray['installed']);
+        $packageArray['isInstalled'] = $isInstalled;
         $packageArray = $this->getVersionInfo($packageArray);
         $packageArray = $this->formatDates($packageArray);
-        $packageArray['iconaction'] = empty($packageArray['installed']) ? 'icon-install' : 'icon-uninstall';
-        $packageArray['textaction'] = empty($packageArray['installed']) ? $this->modx->lexicon('install') : $this->modx->lexicon('uninstall');
+        $packageArray['iconaction'] = !$isInstalled ? 'icon-install' : 'icon-uninstall';
+        $packageArray['textaction'] = !$isInstalled ? $this->modx->lexicon('install') : $this->modx->lexicon('uninstall');
         $packageArray = $this->getPackageMeta($object, $packageArray);
         $packageArray = $this->checkForUpdates($object, $packageArray);