Check for Existence of Cart Before Using

check for the existence of the cart during the is_available() check
before getting the order total (which otherwise will result in a PHP
notice from the admin, etc)
added validation for country code in tax settings, closes #5180
fixed a variable overwritten in tax settings
fixed another variable overwritten in tax settings
Merge pull request #5574 from claudiosmweb/fix-tax-settings-save

[2.1] Fix a variable overwritten in tax settings
Merge pull request #5573 from claudiosmweb/5180-tax-validation

Add validation for country code in tax settings
[2.2] Log changes Closes #4926

Added WC_LOG_DIR constant
Added log viewer
Changed default logging directory to 1 up from ABSPATH inside wc-logs
2 filters for drop down functions Closes #5161
Min
When getting grouped products, don't suppress filters

Closes #4661
stopped "no such file or directory" warnings
Made stateless classes static to allow unhooking of hooks

Closes #5151
introduces the wc_get_log_file_path() function
Additional report filters Closes #4752
Load countries and states when needed rather than in the constructor. Closes #4158
created the language pack upgrader
Subcategory query can now use parent = 0 without breaking

Closes #4644
added post_status filter in WC API, closes #4857
introduces the wc_get_customer_available_downloads() function
created customer api endpoint to get the customer available downloads, closes #4594