Add missing void return type hint

Co-authored-by: philbti <[email protected]>

Author: chadicus

tests/Filter/StringsTest.php

@@ -556,7 +556,7 @@ public function provideRedactFailsOnBadInput() : array
      * @test
      * @dataProvider provideStripEmoji
      */
-    public function stripEmoji(string $input, string $expectedValue)
+    public function stripEmoji(string $input, string $expectedValue): void
     {
         $actualValue = Strings::stripEmoji($input);
         $this->assertSame($expectedValue, $actualValue);