feat: add IoResource and Square key serializer/deserializer tests

modules/io/src/test/scala/de/nowchess/io/json/SquareKeyDeserializerTest.scala

@@ -43,6 +43,7 @@ class SquareKeyDeserializerTest extends AnyFunSuite with Matchers:
     result(Square(File.E, Rank.R4)) shouldBe 3
   }
 
+  // scalafix:off DisableSyntax.null
   test("deserializeKey returns null for invalid square") {
     new SquareKeyDeserializer().deserializeKey("invalid", null) shouldBe null
   }
@@ -58,3 +59,4 @@ class SquareKeyDeserializerTest extends AnyFunSuite with Matchers:
   test("deserializeKey returns null for bad rank") {
     new SquareKeyDeserializer().deserializeKey("e9", null) shouldBe null
   }
+  // scalafix:on DisableSyntax.null