Missing `NA` tag on samples from `NA` nodes in `beacon_api_eth_v1_events_block` table

[cortze]

Description

The dataframes that the Public Xatu database returns when querying for the beacon_api_eth_v1_events_block table include:

• some rows with null values for the meta_client_geo_country_code column
• no data points from nodes in NA*

# Unique Country and Continent codes for the whole set of samples for the last week 
continents:  ['OC', 'AF', 'AS', 'EU', 'SA', None]
countries:  ['BE', 'CZ', 'BR', 'MX', 'IN', 'DE', 'CA', 'FR', 'ZA', 'PT', 'GB', 'CY', 'US', 'AT', 'PL', 'SI', 'SG', 'IT', 'TH', 'FI', 'NZ', 'BG', 'NG', 'KW', 'HU', 'AU']

While taking a closer look at the data, I've realised that some samples coming from nodes located in NA don't have the correct NA meta_client_geo_continent_code tag.

In fact, only nodes from NA countries are getting tagged as null countries:

# List of unique Country codes that have a `null` Continent code  
null continents: ['US', 'CA', 'MX']