Update webpack.ts to match changes made in storybook-addon-next

This issue RyanClementsHax/storybook-addon-next#100 involving storybook-addon-next and the nx plugin @nx/react/plugin/storybook was resolved  RyanClementsHax/storybook-addon-next#101, but that fix didn't make it to this equivalent file here in @storybook/nextjs, making that issue still present when using Storybook 7 and @storybook/nextjs as the framework.

code/frameworks/nextjs/src/images/webpack.ts

@@ -7,6 +7,8 @@ export const configureImages = (baseConfig: WebpackConfig): void => {
   addScopedAlias(baseConfig, 'next/image');
 };
 
+const fallbackFilename = 'static/media/[path][name][ext]'
+
 const configureStaticImageImport = (baseConfig: WebpackConfig): void => {
   const version = getNextjsVersion();
   if (semver.lt(version, '11.0.0')) return;
@@ -30,7 +32,7 @@ const configureStaticImageImport = (baseConfig: WebpackConfig): void => {
       {
         loader: require.resolve('@storybook/nextjs/next-image-loader-stub.js'),
         options: {
-          filename: assetRule.generator?.filename,
+          filename: assetRule.generator?.filename ?? fallbackFilename,
         },
       },
     ],
@@ -40,7 +42,7 @@ const configureStaticImageImport = (baseConfig: WebpackConfig): void => {
     issuer: /\.(css|scss|sass)$/,
     type: 'asset/resource',
     generator: {
-      filename: assetRule.generator?.filename,
+      filename: assetRule.generator?.filename ?? fallbackFilename,
     },
   });
 };