Adjust test for new default values of .fromEnvironment

Breaking change #40678 mandates new default values for
String.fromEnvironment, this CL adjusts the expectations of a test
accordingly.

Change-Id: Ib8a26005673d07c67f65708fc9c4ae7b17421670
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/139811
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Erik Ernst <[email protected]>

Author: eernstg

tests/lib_2/isolate/string_from_environment_default_value_test.dart

@@ -10,7 +10,7 @@ import "dart:isolate";
 import "package:expect/expect.dart";
 
 void test(port) {
-  Expect.isNull(const String.fromEnvironment('NOT_FOUND'));
+  Expect.equals('', const String.fromEnvironment('NOT_FOUND'));
   Expect.equals(
       'x', const String.fromEnvironment('NOT_FOUND', defaultValue: 'x'));
   if (port != null) port.send(null);