fix(ivy): generate empty ngfactory files if needed (#27470)

Previously the ngfactory shim generator in ngtsc would always write two
imports in the factory file shims:

1) an import to @angular/core
2) an import to the base file

If the base file has no exports, import #2 would be empty. This turns out
to cause issues downstream.

This commit changes the generated shim so if there are no exports in the
base file, the generated shim is empty too.

PR Close #27470

packages/compiler-cli/src/ngtsc/shims/src/factory_generator.ts

@@ -47,20 +47,23 @@ export class FactoryGenerator implements ShimGenerator {
                             // Grab the symbol name.
                             .map(decl => decl.name !.text);
 
+    let sourceText = '';
+    if (symbolNames.length > 0) {
       // For each symbol name, generate a constant export of the corresponding NgFactory.
       // This will encompass a lot of symbols which don't need factories, but that's okay
       // because it won't miss any that do.
       const varLines = symbolNames.map(
           name => `export const ${name}NgFactory = new i0.ɵNgModuleFactory(${name});`);
-    const sourceText = [
+      sourceText = [
         // This might be incorrect if the current package being compiled is Angular core, but it's
         // okay to leave in at type checking time. TypeScript can handle this reference via its path
-      // mapping, but downstream bundlers can't. If the current package is core itself, this will be
-      // replaced in the factory transformer before emit.
+        // mapping, but downstream bundlers can't. If the current package is core itself, this will
+        // be replaced in the factory transformer before emit.
         `import * as i0 from '@angular/core';`,
         `import {${symbolNames.join(', ')}} from '${relativePathToSource}';`,
         ...varLines,
       ].join('\n');
+    }
     return ts.createSourceFile(
         genFilePath, sourceText, original.languageVersion, true, ts.ScriptKind.TS);
   }