Add empty values detection and reporting

[ivbeg]

trafficstars

Automate detection of empty values and exclude them from data analysis.

Possible empty values: None, 'N/A', empty string, 'NaN', 'None', '-'

The following actions are required:

• [ ] Add empty values detection during analysis stage
• [ ] Add detected empty values and number of values to analyzer report as empty_values, num_empty