gitlab: compare CIRRUS_nn vars against 'null' not ""

gitlab: compare CIRRUS_nn vars against 'null' not ""

The GitLab variable comparisons don't have shell like semantics where
an unset variable compares equal to empty string. We need to explicitly
test against 'null' to detect an unset variable.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Tested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220608160651.248781-1-berrange@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220613171258.1905715-7-alex.bennee@linaro.org>