If current package folder is null don't delete (#870)

Author: AndrewJack

android/app/src/main/java/com/microsoft/codepush/react/FileUtils.java

@@ -55,6 +55,10 @@ public static void copyDirectoryContents(String sourceDirectoryPath, String dest
     }
 
     public static void deleteDirectoryAtPath(String directoryPath) {
+        if (directoryPath == null) {
+            CodePushUtils.log("deleteDirectoryAtPath attempted with null directoryPath");
+            return;
+        }
         File file = new File(directoryPath);
         if (file.exists()) {
             deleteFileOrFolderSilently(file);