Initializing ExpectException's name to the empty string instead of null when no name is provided.

Change-Id: I2d9d6cafe3e25b1ecf1a9385405262c557887667
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/139325
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Mark Zhou <markzipan@google.com>

pkg/expect/lib/expect.dart

@@ -703,10 +703,10 @@ class ExpectException {
   final String name;
 
   ExpectException(this.message)
-      : name = (_getTestName == null) ? null : _getTestName();
+      : name = (_getTestName == null) ? "" : _getTestName();
 
   String toString() {
-    if (name != null) return 'In test "$name" $message';
+    if (name != "") return 'In test "$name" $message';
     return message;
   }
 }