Excessive log messages converting NULL VARCHAR into INT

There are lots of conversion error messages

error in int conversion from ''

in journalctl output running test001.
The MCOL-4580 patch replaced string to int conversion method with additional stderr message if conversion has a flaw or failed. The approach didn't take into account two facts: empty string represents VARCHAR NULL value, empty string conversion always fails. The combination of two causes excessive log messages.
As a temporary solution I will partially revert the original code that existed before MCOL-4580 and create an issue to start TreeNode refactoring in the context of string to integer types conversion.