don't fail on missing source dirs

buildSrc/src/main/groovy/org/elasticsearch/gradle/precommit/LicenseHeadersTask.groovy

@@ -58,9 +58,12 @@ public class LicenseHeadersTask extends DefaultTask {
                // checks all the java sources (allJava)
                for (SourceSet set : project.sourceSets) {
                    for (File dir : set.allJava.srcDirs) {
+                       // sometimes these dirs don't exist, e.g. site-plugin has no actual java src/main...
+                       if (dir.exists()) {
                            ant.fileset(dir: dir)
                        }
                    }
+               }
 
                // BSD 4-clause stuff (is disallowed below)
                substringMatcher(licenseFamilyCategory: "BSD4 ",