Adjusted InMemoryTest to newest phpstan type inference improvements

Ref: https://github.com/phpstan/phpstan/releases/tag/1.8.3
Ref: phpstan/phpstan-src#1664

test/unit/Signer/Key/InMemoryTest.php

@@ -51,10 +51,7 @@ public function exceptionShouldBeRaisedWhenInvalidBase64CharsAreUsed(): void
      */
     public function base64EncodedShouldDecodeKeyContents(): void
     {
-        $encoded = base64_encode('testing');
-        self::assertNotSame('', $encoded);
-
-        $key = InMemory::base64Encoded($encoded);
+        $key = InMemory::base64Encoded(base64_encode('testing'));
 
         self::assertSame('testing', $key->contents());
     }