Do not fail with empty transactions when using CAS.

lib/Predis.php

@@ -922,7 +922,7 @@ class MultiExecBlock {
                 }
             }
 
-            if ($this->_initialized === false) {
+            if ($this->_initialized === false || count($this->_commands) == 0) {
                 return;
             }
 

test/PredisClientFeatures.php

@@ -569,7 +569,16 @@ class PredisClientFeaturesTestSuite extends PHPUnit_Framework_TestCase {
         $client->flushdb();
 
         $replies = $client->multiExec(function($multi) { });
+        $this->assertEquals(0, count($replies));
+
+        $options = array('cas' => true);
+        $replies = $client->multiExec($options, function($multi) { });
+        $this->assertEquals(0, count($replies));
 
+        $options = array('cas' => true);
+        $replies = $client->multiExec($options, function($multi) {
+            $multi->multi();
+        });
         $this->assertEquals(0, count($replies));
     }