Null definitions on input of Standardization

## Background
`Null` value, an uknown field entry, coming from Src can be sometimes defined by some actual special value. This is not covered by Standardization capabilities yet (except `fixedWidth` input format).

## Goal
Make a definition of `null` value on source possible for Standardization

## Expected Task List
A list of expected issues that will be needed to achieve this Epic
1. Command line `--null-value` for _csv_ format
2. `null_value` metadata for column
3. Command-line option of `--null-value` is not covered by any lineage or other quality assurance mechanism. Perhaps the value should be recorded in _INFO file

The issue names no files, tests, or entry points; begin by locating the CSV standardization command and column-metadata handling in the Scala project. Completion covers the listed scope: a CSV --null-value option, null_value column metadata, and determining whether the option should be recorded in the INFO file.

Scritto dal modello di indicizzazione a partire dal testo della issue.