Don't display anything when the id is invalid.

The previous default of displaying top-level projects when the id is
invalid is just confusing.

Change-Id: If78a54088f7ec4bc68098a7b834a39a576bc641b
diff --git a/tools/report.php b/tools/report.php
index 1628355..0ab1252 100755
--- a/tools/report.php
+++ b/tools/report.php
@@ -188,18 +188,22 @@
 <p>EXPERIMENTAL. This page shows some basic metrics.</p>
 <?php
 
-$project = Project::getProject($id);
-if ($project) {
-	renderProjectReport($project, $options, $showCommitters);
-	if ($showSubprojects) {
-		foreach($project->getChildren() as $child) {
-			renderProjectReport($child, $options, false);
-		}
-	}
-} else {
+if ($id == null) {
 	foreach(Project::getTopLevelProjects() as $project) {
 		renderProjectReport($project, $options, false);
 	}
+} else {
+	$project = Project::getProject($id);
+	if ($project) {
+		renderProjectReport($project, $options, $showCommitters);
+		if ($showSubprojects) {
+			foreach($project->getChildren() as $child) {
+				renderProjectReport($child, $options, false);
+			}
+		}
+	} else {
+		print "<p>There is no project with that ID.</p>";
+	}
 }
 ?>
 </div>