fix(@angular-devkit/build-angular): avoid spawning workers when there are no routes to prerender

This commit fixes an issue were previously we spawned piscina with `maxThreads` set to `0` which causes it to exit with a non zero error code when there are no routes to prerender.

Now, in the application builder we exit at n earlier stage if there are no routes to prerender.

Author: alan-agius4

packages/angular_devkit/build_angular/src/utils/server-rendering/prerender.ts

@@ -37,7 +37,19 @@ export async function prerenderPages(
   warnings: string[];
   errors: string[];
 }> {
+  const output: Record<string, string> = {};
+  const warnings: string[] = [];
+  const errors: string[] = [];
   const allRoutes = await getAllRoutes(tsConfigPath, appShellOptions, prerenderOptions);
+
+  if (allRoutes.size < 1) {
+    return {
+      errors,
+      warnings,
+      output,
+    };
+  }
+
   const outputFilesForWorker: Record<string, string> = {};
 
   for (const { text, path } of outputFiles) {
@@ -65,10 +77,6 @@ export async function prerenderPages(
     ],
   });
 
-  const output: Record<string, string> = {};
-  const warnings: string[] = [];
-  const errors: string[] = [];
-
   try {
     const renderingPromises: Promise<void>[] = [];