Prepare for php 7.2

* allow `|length` of `null`
* allow `|length` of objects not implementing `\Countable`

* To get no errors when running with PHP 7.2 You need to use phpunit
^6.1, otherwise the `each` function, used by phpunit, triggers errors.
You can do so by running phpunit like:
`SYMFONY_PHPUNIT_VERSION=6.1 phpnightly vendor/bin/simple-phpunit` *

lib/Twig/Extension/Core.php

@@ -1264,6 +1264,10 @@ if (function_exists('mb_get_info')) {
      */
     function twig_length_filter(Twig_Environment $env, $thing)
     {
+        if (null === $thing) {
+            return 0;
+        }
+
         if (is_scalar($thing)) {
             return mb_strlen($thing, $env->getCharset());
         }
@@ -1272,7 +1276,11 @@ if (function_exists('mb_get_info')) {
             return mb_strlen((string) $thing, $env->getCharset());
         }
 
-        return count($thing);
+        if ($thing instanceof \Countable || is_array($thing)) {
+            return count($thing);
+        }
+
+        return 1;
     }
 
     /**

test/Twig/Tests/Fixtures/filters/length.legacy.test

@@ -0,0 +1,16 @@
+--TEST--
+"length" filter
+--TEMPLATE--
+{{ null|length }}
+{{ magic|length }}
+{{ non_countable|length }}
+--DATA--
+return array(
+    'null'          => null,                 /* triggers deprecation error */
+    'magic'         => new MagicCallStub(),  /* used to assert we do *not* call __call, also triggers deprecation */
+    'non_countable' => new \StdClass(),      /* triggers deprecation error */
+);
+--EXPECT--
+0
+1
+1

test/Twig/Tests/Fixtures/filters/length.test

@@ -6,7 +6,6 @@
 {{ number|length }}
 {{ to_string_able|length }}
 {{ countable|length }}
-{{ magic|length }}
 --DATA--
 return array(
     'array' => array(1, 4),
@@ -14,7 +13,6 @@ return array(
     'number' => 1000,
     'to_string_able' => new ToStringStub('foobar'),
     'countable' => new CountableStub(42),       /* also asserts we do *not* call __toString() */
-    'magic' => new MagicCallStub(),             /* used to assert we do *not* call __call */
 );
 --EXPECT--
 2
@@ -22,4 +20,3 @@ return array(
 4
 6
 42
-1