Test the PR as follows:

• Applied schema changes
• Then run

show create table piwik_log_visit;
show create table piwik_log_action;
show create table piwik_log_conversion;
show create table piwik_log_conversion_item;
show create table piwik_log_link_visit_action;

• the fields still NOT NULL seem to be all defined in the CoreHome plugin (not de-activable). some fields from log_conversion are defined in Goals and Ecommerce plugin.
• manually tested a Goal conversion and Ecommerce conversions afte de-activating Goal and Ecommerce plugins respectively, and both requests were partially tracked without failing

LGTM