Null values support

[Tomansion]

Is your feature request related to a problem? Please describe. Projects have incomplete (null, NAN, Undefined) values sometime

Describe the solution you'd like

• [x] Allow Null values data insertion
• [x] Analysis to support null values
• [x] Display the percent of null values in analysis page
• [x] Test all widgets with null values
• [x] Update Doc