Fix checking without aggregation on multi-module with no source code in the root project

Author: eyalroth

src/main/groovy/org/scoverage/ScoveragePlugin.groovy

@@ -119,6 +119,7 @@ class ScoveragePlugin implements Plugin<PluginAware> {
 
         project.tasks.create(CHECK_NAME, OverallCheckTask.class) {
             dependsOn(reportTask)
+            onlyIf { extension.reportDir.get().list() }
             group = 'verification'
             coverageType = extension.coverageType
             minimumRate = extension.minimumRate