fix: less aggressive multi-sitemap warnings

Fixes #308

src/module.ts

@@ -106,16 +106,21 @@ export default defineNuxtModule<ModuleOptions>({
     }
 
     // warn about bad config
-    if (!nuxt.options._prepare && Object.keys(config.sitemaps || {}).length) {
-      // if the user is doing multi-sitempas using the sitemaps config, we warn when root keys are used as they won't do anything
-      const invalidRootKeys = [
-        'includeAppSources',
-        'sources',
-      ]
-      for (const key of invalidRootKeys) {
-        if (Object.keys(config).includes(key)) {
-          logger.warn(`You are using multiple-sitemaps but have provided \`sitemap.${key}\` in your Nuxt config. This will be ignored, please move it to the child sitemap config.`)
-          logger.warn('Learn more at: https://nuxtseo.com/sitemap/guides/multi-sitemaps')
+    const normalizedSitemaps = typeof config.sitemaps === 'boolean' ? {} : config.sitemaps || {}
+    if (!nuxt.options._prepare && Object.keys(normalizedSitemaps).length) {
+      // if the only key of config.sitemaps is `index` then we can skip this logic
+      const isSitemapIndexOnly = typeof normalizedSitemaps?.index !== 'undefined' && Object.keys(normalizedSitemaps).length === 1
+      if (!isSitemapIndexOnly) {
+        // if the user is doing multi-sitempas using the sitemaps config, we warn when root keys are used as they won't do anything
+        const invalidRootKeys = [
+          'includeAppSources',
+          'sources',
+        ]
+        for (const key of invalidRootKeys) {
+          if (Object.keys(config).includes(key)) {
+            logger.warn(`You are using multiple-sitemaps but have provided \`sitemap.${key}\` in your Nuxt config. This will be ignored, please move it to the child sitemap config.`)
+            logger.warn('Learn more at: https://nuxtseo.com/sitemap/guides/multi-sitemaps')
+          }
         }
       }
     }