[infra] Avoid crash in test_runner.dart --help

`testConfigurations` expects at least one configuration, so avoid
calling it when there are no configurations.

This fixes the unhandled exception with `test_runner.dart --help`:

```
No build targets found.
Unhandled exception:
RangeError (index): Invalid value: Valid value range is empty: 0
<asynchronous suspension>
```

With this commit, `--help` just prints "No build targets found" and
exits with 0.

This crash was previously fixed in
02a0396f74.

Change-Id: Id82c83f66871c4e811a7502f1e831a0592db2065
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/257580
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Ömer Ağacan <omersa@google.com>

pkg/test_runner/bin/test_runner.dart

@@ -44,6 +44,10 @@ void main(List<String> arguments) async {
     exit(1);
   }
 
+  if (configurations.isEmpty) {
+    return;
+  }
+
   // Run all of the configured tests.
   await testConfigurations(configurations);
 }