Update check-matrix-job.sh to default swift commands (#3237)

Update check-matrix-job.sh to default swift commands so that callers
which don't define them can still run workflows. At the moment they see
failures if they don't define any one of the commands.

Author: rnro

scripts/check-matrix-job.sh

@@ -22,12 +22,12 @@ test -n "${SWIFT_VERSION:-}" || fatal "SWIFT_VERSION unset"
 test -n "${COMMAND:-}" || fatal "COMMAND unset"
 swift_version="$SWIFT_VERSION"
 command="$COMMAND"
-command_5_9="$COMMAND_OVERRIDE_5_9"
-command_5_10="$COMMAND_OVERRIDE_5_10"
-command_6_0="$COMMAND_OVERRIDE_6_0"
-command_6_1="$COMMAND_OVERRIDE_6_1"
+command_5_9="${COMMAND_OVERRIDE_5_9:=""}"
+command_5_10="${COMMAND_OVERRIDE_5_10:=""}"
+command_6_0="${COMMAND_OVERRIDE_6_0:=""}"
+command_6_1="${COMMAND_OVERRIDE_6_1:=""}"
 command_nightly_next="${COMMAND_OVERRIDE_NIGHTLY_NEXT:=""}"
-command_nightly_main="$COMMAND_OVERRIDE_NIGHTLY_MAIN"
+command_nightly_main="${COMMAND_OVERRIDE_NIGHTLY_MAIN:=""}"
 
 if [[ "$swift_version" == "5.9" ]] && [[ -n "$command_5_9" ]]; then
   log "Running 5.9 command override"