Don't abort devstack plugins proposal job due to absence of header/footer

Not having a header or footer should not be a fatal error
especially now that we have removed the footer entirely.

Change-Id: Ibbf3e513b8faa016dc2dac8d11ab4f499b3fc51c
diff --git a/tools/generate-devstack-plugins-list.sh b/tools/generate-devstack-plugins-list.sh
index 8a1f743..be3f60a 100644
--- a/tools/generate-devstack-plugins-list.sh
+++ b/tools/generate-devstack-plugins-list.sh
@@ -41,7 +41,9 @@
 (
 declare -A plugins
 
-test -r data/devstack-plugins-registry.header && cat data/devstack-plugins-registry.header
+if [[ -r data/devstack-plugins-registry.header ]]; then
+    cat data/devstack-plugins-registry.header
+fi
 
 sorted_plugins=$(python tools/generate-devstack-plugins-list.py)
 
@@ -52,7 +54,9 @@
     printf "+----------------------------+-------------------------------------------------------------------------+\n"
 done
 
-test -r data/devstack-plugins-registry.footer && cat data/devstack-plugins-registry.footer
+if [[ -r data/devstack-plugins-registry.footer ]]; then
+    cat data/devstack-plugins-registry.footer
+fi
 ) > doc/source/plugin-registry.rst
 
 if [[ -n ${1} ]]; then