core: fix deobfuscation when class is in the root package (package path is empty)

jadx-core/src/main/java/jadx/core/deobf/PackageNode.java

@@ -61,11 +61,16 @@ public class PackageNode {
 		if (cachedPackageFullAlias == null) {
 			Stack<PackageNode> pp = getParentPackages();
 			StringBuilder result = new StringBuilder();
+
+			if (pp.size() > 0) {
 				result.append(pp.pop().getAlias());
 				while (pp.size() > 0) {
 					result.append(SEPARATOR_CHAR);
 					result.append(pp.pop().getAlias());
 				}
+			} else {
+				result.append(this.getAlias());
+			}
 			cachedPackageFullAlias = result.toString();
 		}
 		return cachedPackageFullAlias;