Fix some "URI->alter(X, null)" callsites

Summary:
Ref T13250. This internally calls `replaceQueryParam(X, null)` now, which fatals if the second parameter is `null`. I hit these legitimately, but I'll look for more callsites and follow up by either allowing this, removing `alter()`, fixing the callsites, or some combination.

(I'm not much of a fan of `alter()`.)

Test Plan: Browsing a paginated list no longer complains about URI construction.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13250

Differential Revision: https://secure.phabricator.com/D20162

src/view/control/AphrontCursorPagerView.php

@@ -99,9 +99,9 @@ final class AphrontCursorPagerView extends AphrontView {
       return null;
     }
 
-    return $this->uri
-      ->alter('before', null)
-      ->alter('after', null);
+    return id(clone $this->uri)
+      ->removeQueryParam('after')
+      ->removeQueryParam('before');
   }
 
   public function getPrevPageURI() {
@@ -113,9 +113,9 @@ final class AphrontCursorPagerView extends AphrontView {
       return null;
     }
 
-    return $this->uri
-      ->alter('after', null)
-      ->alter('before', $this->prevPageID);
+    return id(clone $this->uri)
+      ->removeQueryParam('after')
+      ->replaceQueryParam('before', $this->prevPageID);
   }
 
   public function getNextPageURI() {
@@ -127,9 +127,9 @@ final class AphrontCursorPagerView extends AphrontView {
       return null;
     }
 
-    return $this->uri
-      ->alter('after', $this->nextPageID)
-      ->alter('before', null);
+    return id(clone $this->uri)
+      ->replaceQueryParam('after', $this->nextPageID)
+      ->removeQueryParam('before');
   }
 
   public function render() {