fix(plugin-legacy): empty base makes import fail (fixes #4212) (#8387)

Author: sapphi-red

Diff for: packages/plugin-legacy/src/index.ts

@@ -419,6 +419,8 @@ function viteLegacyPlugin(options: Options = {}): Plugin[] {
         chunk.facadeModuleId
       )
       if (legacyEntryFilename) {
+        // `assets/foo.js` means importing "named register" in SystemJS
+        const nonBareBase = config.base === '' ? './' : config.base
         tags.push({
           tag: 'script',
           attrs: {
@@ -427,7 +429,7 @@ function viteLegacyPlugin(options: Options = {}): Plugin[] {
             // script content will stay consistent - which allows using a constant
             // hash value for CSP.
             id: legacyEntryId,
-            'data-src': config.base + legacyEntryFilename
+            'data-src': nonBareBase + legacyEntryFilename
           },
           children: systemJSInlineCode,
           injectTo: 'body'