Mark sessions as "signed all documents" when Legalpad has been uninstalled

See PHI238. When an install uninstalls "Legalpad", we were incorrectly failing
to mark sessions as "Signed All Required Documents" by bailing early.

Test Plan: Uninstalled Legalpad, logged in.

src/applications/base/controller/PhabricatorController.php

@@ -562,14 +562,14 @@ abstract class PhabricatorController extends AphrontController {
       return null;
     }
 
+    $must_sign_docs = array();
+    $sign_docs = array();
+
     $legalpad_class = 'PhabricatorLegalpadApplication';
     $legalpad_installed = PhabricatorApplication::isClassInstalledForViewer(
       $legalpad_class,
       $viewer);
-    if (!$legalpad_installed) {
-      return null;
-    }
-
+    if ($legalpad_installed) {
       $sign_docs = id(new LegalpadDocumentQuery())
         ->setViewer($viewer)
         ->withSignatureRequired(1)
@@ -577,12 +577,12 @@ abstract class PhabricatorController extends AphrontController {
         ->setOrder('oldest')
         ->execute();
 
-    $must_sign_docs = array();
       foreach ($sign_docs as $sign_doc) {
         if (!$sign_doc->getUserSignature($viewer->getPHID())) {
           $must_sign_docs[] = $sign_doc;
         }
       }
+    }
 
     if (!$must_sign_docs) {
       // If nothing needs to be signed (either because there are no documents