[java] Fix same package import from default package

- If the package was passed as an empty string rather than a `null`, we
    would attempt to load classes such as `.Foo` rather than `Foo`.

pmd-java/src/main/java/net/sourceforge/pmd/lang/java/symboltable/TypeSet.java

@@ -215,7 +215,7 @@ public class TypeSet {
          */
         public CurrentPackageResolver(PMDASMClassLoader pmdClassLoader, String pkg) {
             super(pmdClassLoader);
-            if (pkg == null) {
+            if (pkg == null || pkg.length() == 0) {
                 this.pkg = null;
             } else {
                 this.pkg = pkg + ".";