fix(compiler-cli): do not emit invalid .metadata.json files

If no metadata is collected the `ngc` would generate file
that contained `[null]` instead of eliding the `.metadata.json`
file.

Fixes: #20479

packages/compiler-cli/src/transformers/program.ts

@@ -336,9 +336,11 @@ class AngularCompilerProgram implements Program {
         if (!sf.isDeclarationFile && !GENERATED_FILES.test(sf.fileName)) {
           metadataJsonCount++;
           const metadata = this.metadataCache.getMetadata(sf);
-          const metadataText = JSON.stringify([metadata]);
+          if (metadata) {
-          const outFileName = srcToOutPath(sf.fileName.replace(/\.tsx?$/, '.metadata.json'));
+            const metadataText = JSON.stringify([metadata]);
-          this.writeFile(outFileName, metadataText, false, undefined, undefined, [sf]);
+            const outFileName = srcToOutPath(sf.fileName.replace(/\.tsx?$/, '.metadata.json'));
+            this.writeFile(outFileName, metadataText, false, undefined, undefined, [sf]);
+          }
         }
       });
     }

packages/compiler-cli/test/ngc_spec.ts

@@ -1474,6 +1474,26 @@ describe('ngc transformer command-line', () => {
   });
 
   describe('regressions', () => {
+    //#20479
+    it('should not generate an invalid metadata file', () => {
+      write('src/tsconfig.json', `{
+        "extends": "../tsconfig-base.json",
+        "files": ["lib.ts"],
+        "angularCompilerOptions": {
+          "skipTemplateCodegen": true
+        }
+      }`);
+      write('src/lib.ts', `
+        export namespace A{
+          export class C1 {
+          }
+          export interface I1{
+          }
+        }`);
+      expect(main(['-p', path.join(basePath, 'src/tsconfig.json')])).toBe(0);
+      shouldNotExist('src/lib.metadata.json');
+    });
+
     //#19544
     it('should recognize @NgModule() directive with a redundant @Injectable()', () => {
       write('src/tsconfig.json', `{