🧪 Gracefully handle no-report pytest runs @ tox

Otherwise, it was trying to access argument values that weren't there.

tox.ini

@@ -54,19 +54,23 @@ commands_post =
       os.getenv("GITHUB_ACTIONS") == "true" or sys.exit(); \
       cov_report_arg_prefix = "--cov-report=xml:"; \
       test_report_arg_prefix = "--junitxml="; \
-      cov_report_file = [\
+      cov_reports = [\
         arg[len(cov_report_arg_prefix):] for arg in sys.argv \
         if arg.startswith(cov_report_arg_prefix)\
-      ][-1]; \
-      test_report_file = [\
+      ]; \
+      test_reports = [\
         arg[len(test_report_arg_prefix):] for arg in sys.argv \
         if arg.startswith(test_report_arg_prefix)\
-      ][-1]; \
+      ]; \
+      cov_report_file = cov_reports[-1] if cov_reports else None; \
+      test_report_file = test_reports[-1] if test_reports else None; \
       gh_output_fd = open(\
         os.environ["GITHUB_OUTPUT"], encoding="utf-8", mode="a",\
       ); \
-      print(f"cov-report-files={cov_report_file !s}", file=gh_output_fd); \
-      print(f"test-result-files={test_report_file !s}", file=gh_output_fd); \
+      cov_report_file and \
+        print(f"cov-report-files={cov_report_file !s}", file=gh_output_fd); \
+      test_report_file and \
+        print(f"test-result-files={test_report_file !s}", file=gh_output_fd); \
       print("codecov-flags=pytest", file=gh_output_fd); \
       gh_output_fd.close()' \
     {posargs}