bug #2433 Fix #2431 (mpdude)

This PR was merged into the 1.x branch.

Discussion
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Fix #2431

See title

Commits
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a5c2da6d test if is_object before calling method_exists, fixes #2431

lib/Twig/Extension/Core.php

@@ -1425,7 +1425,7 @@ function twig_test_empty($value)
         return 0 == count($value);
     }
 
-    if (method_exists($value, '__toString')) {
+    if (is_object($value) && method_exists($value, '__toString')) {
         return '' === (string) $value;
     }