fix(use-roadiz-page-title): siteName query (#607)

* fix: return null instead of empty vnode when tag is not defined

* fix: use runtime-config outside the siteName computed to avoid error on runtime

Author: willybrauner

app/composables/use-roadiz-page-title.ts

@@ -2,7 +2,8 @@
 
 export function useRoadizPageTitle(title?: MaybeRefOrGetter<string>) {
     const commonContent = useCommonContent()
-    const siteName = computed(() => commonContent.data.value?.head?.siteName || useRuntimeConfig().public.site.name)
+    const runtimeConfig = useRuntimeConfig()
+    const siteName = computed(() => commonContent.data.value?.head?.siteName || runtimeConfig?.public?.site?.name)
 
     const { searchParamsLabel } = useCurrentPageSearchParams()