ENGCOM-8835: Ensure string is returned when applying filter to email template #32298

 - Merge Pull Request #32298 from Skullsneeze/magento2:bugfix/email-tempalte-filter-not-returning-string
 - Merged commits:
   1. d6cba47
   2. 06412dc
   3. 28e0124
   4. 4b46634
   5. db048d2

Author: magento-engcom-team

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

@@ -1086,7 +1086,7 @@ 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);
         }

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);
+        $this->assertTrue(is_string($filteredValue));
+    }
+
     /**
      * Test basic usages of applyInlineCss
      *