Attempt to fix deprecation notices in StaticController, a=chris

diff --git a/src/controllers/StaticController.php b/src/controllers/StaticController.php
index c69f114ad..2f17e1cc8 100644
--- a/src/controllers/StaticController.php
+++ b/src/controllers/StaticController.php
@@ -205,6 +205,7 @@ EOD;
                 $head_info['page_type'] != 'presentation') {
             $page_header = $group_model->getPageInfoByName(C\PUBLIC_GROUP_ID,
                 $head_info['page_header'], $locale_tag, "read");
+            $page_header ??= [];
             if (isset($page_header['PAGE'])) {
                 $header_parts =
                     explode("END_HEAD_VARS", $page_header['PAGE']);
@@ -217,6 +218,7 @@ EOD;
                 $head_info['page_type'] != 'presentation') {
             $page_footer = $group_model->getPageInfoByName(C\PUBLIC_GROUP_ID,
                 $head_info['page_footer'], $locale_tag, "read");
+            $page_footer ??= [];
             if (isset($page_footer['PAGE'])) {
                 $footer_parts =
                     explode("END_HEAD_VARS", $page_footer['PAGE']);