fix: resolve PHPstan errors

app/Rules/IsValidActionExpression.php

@@ -21,6 +21,8 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+declare(strict_types=1);
+
 namespace FireflyIII\Rules;
 
 use Closure;

app/TransactionRules/Expressions/ActionExpression.php

@@ -133,7 +133,7 @@ class ActionExpression
         return $this->validationError === null;
     }
 
-    public function getValidationError()
+    public function getValidationError(): ?SyntaxError
     {
         return $this->validationError;
     }

app/TransactionRules/Expressions/ActionExpressionLanguageProvider.php

@@ -20,6 +20,8 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+declare(strict_types=1);
+
 namespace FireflyIII\TransactionRules\Expressions;
 
 use Symfony\Component\ExpressionLanguage\ExpressionFunction;