Null values support

[Tomansion]

Describe your changes

• The columns can now have missing values inside them
• Checked that all widgets support columns with missing values
  • Parallel coordinates null values line is missing (cf images)
  • Distribution plot shows “null” for classes, ignores values for numbers
  • Point plot scatter don't displays points with missing data
  • Statistics ignore missing values and display % of missing values
  • Range slider hides missing points, same for the background
  • Interval plot works, but the shape is broken (won't fix)
  • Correlation matrix plot null values are considered as 0 values
  • Confusion matrix plot null values are considered as 0 values
  • Density plot ignores null values
  • Night stars null values are considered as 0 values
  • Sample Array displays the null values properly
• We can now filter values on null values
• Added percentage of missing values in columns

Screenshots (if appropriate):

Issue ticket number and link

Checklist before requesting a review

• I have performed a self-review of my code
• I have increased the version number in the frontend/package.json and backend/swagger.yaml files
• I have checked that Black, Flake8, Prettier and Cspell are passing