Clean null values and duplicates

📂 Task: Identify and remove any null values and duplicate rows in the dataset.
📌 File to modify: main.ipynb

✅ Expected Output:

Use df.isnull().sum() to check for nulls
Use dropna() and drop_duplicates()
Print shape of dataset before and after cleaning

💡 Keep your code clean and well-commented.

The work is in main.ipynb. Start by loading the dataset, then use df.isnull().sum() to check for nulls, and df.dropna() and df.drop_duplicates() to clean. Print the shape before and after cleaning. Ensure the code is clean and well-commented as requested.

由索引模型根据 Issue 内容生成。