Add Optional() method to MatchQuery

[chrismarget-j]

• Bugfix slice out-of-bound terror (lol typo) when a path query element has no attributes: node() would crash, but node(type='system', label='foo') was okay
• Eliminate use of positional struct elements in test. This probably broke in Reintroduce CT primitive label feature #103. These are the changes in apstra/two_stage_l3_clos_connectivity_template_integration_test.go.
• Add optional element to PathQuery and MatchQuery structs, so they know how to string-ify in match+optional context
• Change MatchQuery to it receives QEQuery implementations rather than only PathQuery objects. Now we can write queries with match(match())
• Introduce MatchQuery.Optional(), which sets the optional flag on child queries
• Test