View filtering all records with no argument, empty text with invalid argument

I have a view with a block display that I need to filter by:

• All records when no argument present e.g. /foo
• All nodes with specified valid taxonomy term ID e.g. /foo/bar
• Show empty text when taxonomy term ID is attached to no content e.g. /foo/baz

• WHEN THE FILTER VALUE IS NOT AVAILABLE is set to Display all results for the specified field.
• WHEN THE FILTER VALUE IS AVAILABLE OR A DEFAULT IS PROVIDED is set to Specify validation criteria, Taxonomy Term.
• Filter value type is set to term name converted to term ID.
• Transform dashes in URL to spaces in term name filter values.
• Action to take if filter value does not validate is set to Display contents of "No results found".

• The view preview is working correctly, adding taxonomy_term_data_field_data_field_VOCAB.tid = 'xx' to the query but on the front-end it doesn’t appear to be working at all.
• I have Global: Null as the first entry under Contextual filters as the argument needed to filter is the second argument in the URL string.