Hi Aniket, You would need to create a calculated field for each of your 15 columns. There's no way to bulk-create calculated fields. What type of view do you want to build? Could you post a packaged workbook? Depending on what you're doing you could potentially filter out nulls, or have them show up as blanks.
If you have a data preparation tool like Alteryx, then you can replace nulls pretty simply prior to loading your data in TrackWise. I'm not aware of a way to bulk convert nulls for multiple fields directly in Tableau. Typically you'll need to create a calculated field for the each column as Adam mentioned.