Fixed 'PackageInternalsFinder.fuse' throws an exception when jar in jar is empty (#2941)

memorycompiler/src/main/java/com/taobao/arthas/compiler/PackageInternalsFinder.java

@@ -98,6 +98,9 @@ public class PackageInternalsFinder {
 
             JarURLConnection jarConn = (JarURLConnection) packageFolderURL.openConnection();
             String rootEntryName = jarConn.getEntryName();
+
+            if (rootEntryName != null) {
+                //可能为 null（自己没有类文件时）
                 int rootEnd = rootEntryName.length() + 1;
 
                 Enumeration<JarEntry> entryEnum = jarConn.getJarFile().entries();
@@ -112,6 +115,7 @@ public class PackageInternalsFinder {
                         result.add(new CustomJavaFileObject(binaryName, uri));
                     }
                 }
+            }
         } catch (Exception e) {
             throw new RuntimeException("Wasn't able to open " + packageFolderURL + " as a jar file", e);
         }