fix: don't override empty public folder (#3513)

packages/netlify-cms-core/src/actions/__tests__/config.spec.js

@@ -57,6 +57,16 @@ describe('config', () => {
             }),
           ).get('public_folder'),
         ).toEqual('/publib/path');
+        expect(
+          applyDefaults(
+            fromJS({
+              foo: 'bar',
+              media_folder: 'path/to/media',
+              public_folder: '',
+              collections: [],
+            }),
+          ).get('public_folder'),
+        ).toEqual('');
       });
     });
 

packages/netlify-cms-core/src/actions/config.js

@@ -39,7 +39,7 @@ export function applyDefaults(config) {
 
       // Use media_folder as default public_folder.
       const defaultPublicFolder = `/${trimStart(map.get('media_folder'), '/')}`;
-      if (!map.get('public_folder')) {
+      if (!map.has('public_folder')) {
         map.set('public_folder', defaultPublicFolder);
       }