Report when custom variable values are empty, grouped in "Value not defined" #2676

We should report the custom variable values that are empty and group them under a "Not defined" category.

Also, let's make sure that "Product category not defined" for ecommerce items rows for Ecommerce views metrics (visits,actions,etc.) are reported in the Ecommerce reports as a result.

(In [5203]) Fixes #2676

• Now tracking custom variables with empty value in setEcommerceView
• Now aggregating custom variables with empty value under a label "Value not defined" (in english always)
• Updated tests and refactored code in API instead of archiving