fix(deobf): resolve NPE when package is empty (if rename is disabled)

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

@@ -31,14 +31,17 @@ public class PackageNode {
 	public String getFullName() {
 		if (cachedPackageFullName == null) {
 			Deque<PackageNode> pp = getParentPackages();
-
-			StringBuilder result = new StringBuilder();
-			result.append(pp.pop().getName());
-			while (!pp.isEmpty()) {
-				result.append(SEPARATOR_CHAR);
+			if (pp.isEmpty()) {
+				cachedPackageFullName = "";
+			} else {
+				StringBuilder result = new StringBuilder();
 				result.append(pp.pop().getName());
+				while (!pp.isEmpty()) {
+					result.append(SEPARATOR_CHAR);
+					result.append(pp.pop().getName());
+				}
+				cachedPackageFullName = result.toString();
 			}
-			cachedPackageFullName = result.toString();
 		}
 		return cachedPackageFullName;
 	}