Do not warn when triggered job is a success (#375)

I keep having log messages such as:

  WARNING: [hudson.plugins.parameterizedtrigger.TriggerBuilder perform]
  Attempting to use the result of unfinished build <name of job> #1234

The message was introduced in #161 by 06d8f19 but in #148 / 02ee6ea,
mapBuildResult() was made to return `null` by default (eg on SUCCESS).

The resut is the warning is always triggered when the triggered build
succeeds.

Fixes https://phabricator.wikimedia.org/T336782

Co-authored-by: Mark Waite <mark.earl.waite@gmail.com>
Co-authored-by: Bruno Verachten <gounthar@gmail.com>

src/main/java/hudson/plugins/parameterizedtrigger/TriggerBuilder.java

@@ -187,11 +187,8 @@ public boolean perform(AbstractBuild<?, ?> build, Launcher launcher, BuildListen
                                     if (buildStepResult
                                             && config.getBlock().mapBuildStepResult(completedRun.getResult())) {
                                         Result r = config.getBlock().mapBuildResult(completedRun.getResult());
-                                        if (r != null) {
+                                        if (r != null) { // The blocking job is not a success
                                             build.setResult(r);
-                                        } else {
-                                            LOGGER.warning("Attempting to use the result of unfinished build "
-                                                    + completedRun.toString());
                                         }
                                     } else {
                                         buildStepResult = false;