Misc return type fixes

Summary:
Fix a few issues raised by static code analysis:

* /src/applications/audit/query/DiffusionInternalCommitSearchEngine.php:39 Method DiffusionInternalCommitSearchEngine::getURI() should return string but returns null.
* /src/applications/diviner/storage/DivinerLiveSymbol.php:250 Method DivinerLiveSymbol::getMarkupText() should return string but empty return statement found.
* /src/applications/phame/search/PhamePostFerretEngine.php:15 Method PhamePostFerretEngine::newSearchEngine() should return string but returns PhamePostSearchEngine.
* /src/applications/phame/storage/PhamePost.php:270 Method PhamePost::getMarkupText() should return string but returns null.
* /src/applications/policy/rule/PhabricatorPolicyRule.php:188 Method PhabricatorPolicyRule::getObjectPolicyKey() should return string but returns null.
* /src/applications/project/query/PhabricatorProjectColumnSearchEngine.php:45 Method PhabricatorProjectColumnSearchEngine::getURI() should return string but returns null.
* /src/applications/transactions/feed/PhabricatorApplicationTransactionFeedStory.php:124 Method PhabricatorApplicationTransactionFeedStory::getMarkupText() should return string but returns null.

Test Plan: Read the code, run static code analysis.

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D26320

src/applications/audit/query/DiffusionInternalCommitSearchEngine.php

···
   }
 
   protected function getURI($path) {
-    return null;
+    return '';
   }
 
   protected function renderResultList(

src/applications/diviner/storage/DivinerLiveSymbol.php

···
 
   public function getMarkupText($field) {
     if (!$this->getAtom()) {
-      return;
+      return '';
     }
 
     return $this->getAtom()->getDocblockText();

src/applications/phame/storage/PhamePost.php

···
       case self::MARKUP_FIELD_BODY:
         return $this->getBody();
     }
-    return null;
+    return '';
   }
 
   public function didMarkupText(

src/applications/policy/rule/PhabricatorPolicyRule.php

···
    * @task objectpolicy
    */
   public function getObjectPolicyKey() {
-    return null;
+    return '';
   }
 
   final public function getObjectPolicyFullKey() {

src/applications/project/query/PhabricatorProjectColumnSearchEngine.php

···
   protected function getURI($path) {
     // NOTE: There's no way to query columns in the web UI, at least for
     // the moment.
-    return null;
+    return '';
   }
 
   protected function getBuiltinQueryNames() {

src/applications/search/engine/PhabricatorApplicationSearchEngine.php

···
    * Return the URI to a path within the application. Used to construct default
    * URIs for management and results.
    *
-   * @return string URI to path.
+   * @return string URI to path; empty string if not implemented or applicable.
    * @task uri
    */
   abstract protected function getURI($path);

src/applications/transactions/feed/PhabricatorApplicationTransactionFeedStory.php

···
       }
     }
 
-    return null;
+    return '';
   }
 
   public function renderView() {