Dont quote realm if it's null in Credentials.toString

ivy/src/main/scala/sbt/librarymanagement/ivy/Credentials.scala

@@ -88,5 +88,11 @@ final class DirectCredentials(
     val userName: String,
     val passwd: String
 ) extends Credentials {
-  override def toString = s"""DirectCredentials("$realm", "$host", "$userName", ****)"""
+  override def toString = {
+    val dq = '"'
+    val r =
+      if (realm == null) "null"
+      else dq + realm + dq
+    s"""DirectCredentials($r, "$host", "$userName", ****)"""
+  }
 }

ivy/src/test/scala/sbt/internal/librarymanagement/CredentialsSpec.scala

@@ -25,4 +25,16 @@ class CredentialsSpec extends AnyFunSuite {
 
     credentialsFile.delete()
   }
+
+  test("DirectCredentials.toString") {
+    assert(
+      Credentials(realm = null, host = "example.org", userName = "username", passwd = "password").toString ==
+        """DirectCredentials(null, "example.org", "username", ****)"""
+    )
+
+    assert(
+      Credentials(realm = "realm", host = "example.org", userName = "username", passwd = "password").toString ==
+        """DirectCredentials("realm", "example.org", "username", ****)"""
+    )
+  }
 }