ZF-10706: fix signals return issue

- Signals should always return a ResponseCollection
- ResponseCollection::last() should return null when empty
- Added test for behavior described in issue

test/ContainerTest.php

@@ -493,4 +493,14 @@ public function testIterationHonorsExpirationTimestamps()
         }
         $this->assertSame($expected, $test);
     }
+
+    /**
+     * @group ZF-10706
+     */
+    public function testValidationShouldNotRaiseErrorForMissingResponseObject()
+    {
+        $session = new Container('test');
+        $session->test = 42;
+        $this->assertEquals(42, $session->test);
+    }
 }