Moved string withing catch block, and added unit test scenario

Author: Martijn Swinkels

app/code/Magento/Email/Model/Template/Filter.php

@@ -1086,10 +1086,10 @@ public function filter($value)
             if ($this->_appState->getMode() == \Magento\Framework\App\State::MODE_DEVELOPER) {
                 $value = sprintf(__('Error filtering template: %s'), $e->getMessage());
             } else {
-                $value = __("We're sorry, an error has occurred while generating this content.");
+                $value = (string) __("We're sorry, an error has occurred while generating this content.");
             }
             $this->_logger->critical($e);
         }
-        return (string) $value;
+        return $value;
     }
 }

app/code/Magento/Email/Test/Unit/Model/Template/FilterTest.php

@@ -271,6 +271,16 @@ protected function getModel($mockedMethods = null)
             ->getMock();
     }
 
+    /**
+     * Test exception handling of filter method
+     */
+    public function testFilterExceptionHandler()
+    {
+        $filter = $this->getModel();
+        $filteredValue = $filter->filter(null);
+        $this->assertInternalType('string', $filteredValue);
+    }
+
     /**
      * Test basic usages of applyInlineCss
      *