Fix trailing slash for default image loader (#18298)

Co-authored-by: Nkodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

packages/next/next-server/server/config.ts

@@ -218,18 +218,12 @@ function assignDefaults(userConfig: { [key: string]: any }) {
   if (result?.images) {
     const { images } = result
 
-    // Normalize defined image host to end in slash
-    if (images?.path) {
-      if (images.path[images.path.length - 1] !== '/') {
-        images.path += '/'
-      }
-    }
-
     if (typeof images !== 'object') {
       throw new Error(
         `Specified images should be an object received ${typeof images}`
       )
     }
+
     if (images.domains) {
       if (!Array.isArray(images.domains)) {
         throw new Error(
@@ -306,6 +300,14 @@ function assignDefaults(userConfig: { [key: string]: any }) {
         )
       }
     }
+
+    // Append trailing slash for non-default loaders
+    if (images.path) {
+      const isDefaultLoader = !images.loader || images.loader === 'default'
+      if (!isDefaultLoader && images.path[images.path.length - 1] !== '/') {
+        images.path += '/'
+      }
+    }
   }
 
   if (result.experimental?.i18n) {

test/integration/image-component/default/test/index.test.js

@@ -71,10 +71,7 @@ function runTests(mode) {
       expect(
         await hasImageMatchingUrl(
           browser,
-          mode === 'serverless'
-            ? // FIXME: this is a bug
-              `http://localhost:${appPort}/_next/image/?url=%2Ftest.jpg&w=420&q=75`
-            : `http://localhost:${appPort}/_next/image?url=%2Ftest.jpg&w=420&q=75`
+          `http://localhost:${appPort}/_next/image?url=%2Ftest.jpg&w=420&q=75`
         )
       ).toBe(true)
     } finally {