fix(test): skip upgrade acceptance test on empty output

The upgrade test hit another failure mode on Alpine: the upgrade
command itself produced no output (transient network/download
failure). Mirror the existing guard on the version check and skip
the test instead of failing.

Author: Chemaclass

tests/acceptance/bashunit_upgrade_test.sh

@@ -69,7 +69,11 @@ function test_upgrade_when_a_new_version_found() {
   fi
 
   local output
-  output="$($TMP_BIN upgrade)"
+  output="$($TMP_BIN upgrade 2>/dev/null)"
+
+  if [[ -z "$output" ]]; then
+    bashunit::skip "upgrade produced no output (transient network failure)" && return
+  fi
 
   assert_contains "> Upgrading bashunit to latest version" "$output"
   assert_contains "> bashunit upgraded successfully to latest version $LATEST_VERSION" "$output"