tests/docker: check for an parameters not empty string

Reported-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

Message-Id: <20200701135652.1366-17-alex.bennee@linaro.org>

tests/docker/common.rc

@@ -47,7 +47,7 @@ build_qemu()
 check_qemu()
 {
     # default to make check unless the caller specifies
-    if test -z "$@"; then
+    if [ $# = 0 ]; then
         INVOCATION="check"
     else
         INVOCATION="$@"