Numeric columns should not use 0 when no value (#323)

Adding additional clarity to ensure numeric columns do not use `0` in
place of a null when there is no value.

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Co-authored-by: Irena Jurica <irena.jurica@neos.hr>

specification/attributes/null_handling.md

@@ -20,7 +20,7 @@ Indicates how to handle columns that don't have a value.
 ## Requirements
 
 * Columns MUST use NULL when there isn't a value that can be specified for a nullable column.
-* Columns MUST NOT use empty strings or placeholder values such as "Not Set" or "Not Applicable" in columns, regardless of if the column allows nulls or not.
+* Columns MUST NOT use empty strings or placeholder values such as 0 for numeric columns or "Not Applicable" for string columns, regardless of whether the column allows nulls or not.
 
 ## Exceptions