Sharepoint handle null values

[Rafalz13]

Summary

Added list of na_values. NA value is out of the list to treat NA as a string value.
Changed the way of casting column to string to be able to generate NaN instead of nan string values.

Importance

The change was needed to handle NaN values after casting to string. In the current version all values (including nan) are converted to string.

Checklist

This PR:

• follows the guidelines laid out in CONTRIBUTING.md
• links relevant issue(s)
• adds/updates tests (if appropriate)
• adds/updates docstrings (if appropriate)
• adds an entry in CHANGELOG.md

[Rafalz13]

The change is tested using separate docker images.