Automatically handle null values

[aleksbobic]

Is your feature request related to a problem? Please describe.
When uploading a dataset the users should be provided with an option to fill the null values automatically.

Describe the solution you'd like
When a dataset is provided to CSX it should automatically detect columns with null values. Based on the type of the column it should provide a few suggestions for null value alternatives as well as a text field asking the user to enter a specific value they would like to use instead of a suggestion.