Avoid StringIndexOutOfBoundsException when .java files were found

directly inside one of the source directories (classes in the default
package).


git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@267961 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/taskdefs/Javadoc.java

@@ -905,7 +905,7 @@ public class Javadoc extends Task {
     			}
     		});
     
-    	if (foundJavaFile) {
+    	if (foundJavaFile && vpath.length() > 0) {
     		String newPkg = vpath.substring(1).replace(File.separatorChar, '.');
     		if (!pkgs.contains(newPkg)) {
     			pkgs.addElement(newPkg);