fix(ivy): don't generate code for blank NgModule fields (angular#28387)

Currently `compileNgModule` generates an empty array for optional fields that are omitted from an `NgModule` declaration (e.g. `bootstrap`, `exports`). This isn't necessary, because `defineNgModule` has some code to default these fields to empty arrays at runtime if they aren't defined. The following changes will only output code if there are values for the particular field.

PR Close angular#28387

Author: crisbeto

packages/compiler-cli/test/ngtsc/ngtsc_spec.ts

@@ -275,7 +275,7 @@ describe('ngtsc behavioral tests', () => {
     expect(jsContents)
         .toContain(
             'i0.ɵdefineNgModule({ type: TestModule, bootstrap: [TestCmp], ' +
-            'declarations: [TestCmp], imports: [], exports: [] })');
+            'declarations: [TestCmp] })');
 
     const dtsContents = env.getContents('test.d.ts');
     expect(dtsContents)
@@ -373,7 +373,7 @@ describe('ngtsc behavioral tests', () => {
     env.tsconfig();
     env.write('test.ts', `
         import {Component, NgModule} from '@angular/core';
-        
+
         export class Dep {}
 
         export class Token {
@@ -1562,24 +1562,24 @@ describe('ngtsc behavioral tests', () => {
       env.tsconfig();
       env.write('node_modules/external/index.d.ts', `
         import {ɵDirectiveDefWithMeta, ɵNgModuleDefWithMeta} from '@angular/core';
-  
+
         export declare class ExternalDir {
           static ngDirectiveDef: ɵDirectiveDefWithMeta<ExternalDir, '[test]', never, never, never, never>;
         }
-  
+
         export declare class ExternalModule {
           static ngModuleDef: ɵNgModuleDefWithMeta<ExternalModule, [typeof ExternalDir], never, [typeof ExternalDir]>;
         }
       `);
       env.write('test.ts', `
         import {Component, Directive, NgModule} from '@angular/core';
         import {ExternalModule} from 'external';
-  
+
         @Component({
           template: '<div test></div>',
         })
         class Cmp {}
-  
+
         @NgModule({
           declarations: [Cmp],
           // Multiple imports of the same module used to result in duplicate directive references

packages/compiler/src/render3/r3_module_compiler.ts

@@ -64,14 +64,34 @@ export interface R3NgModuleMetadata {
  */
 export function compileNgModule(meta: R3NgModuleMetadata): R3NgModuleDef {
   const {type: moduleType, bootstrap, declarations, imports, exports} = meta;
-  const expression = o.importExpr(R3.defineNgModule).callFn([mapToMapExpression({
-    type: moduleType,
-    bootstrap: o.literalArr(bootstrap.map(ref => ref.value)),
-    declarations: o.literalArr(declarations.map(ref => ref.value)),
-    imports: o.literalArr(imports.map(ref => ref.value)),
-    exports: o.literalArr(exports.map(ref => ref.value)),
-  })]);
-
+  const definitionMap = {
+    type: moduleType
+  } as{
+    type: o.Expression,
+    bootstrap: o.LiteralArrayExpr,
+    declarations: o.LiteralArrayExpr,
+    imports: o.LiteralArrayExpr,
+    exports: o.LiteralArrayExpr
+  };
+
+  // Only generate the keys in the metadata if the arrays have values.
+  if (bootstrap.length) {
+    definitionMap.bootstrap = o.literalArr(bootstrap.map(ref => ref.value));
+  }
+
+  if (declarations.length) {
+    definitionMap.declarations = o.literalArr(declarations.map(ref => ref.value));
+  }
+
+  if (imports.length) {
+    definitionMap.imports = o.literalArr(imports.map(ref => ref.value));
+  }
+
+  if (exports.length) {
+    definitionMap.exports = o.literalArr(exports.map(ref => ref.value));
+  }
+
+  const expression = o.importExpr(R3.defineNgModule).callFn([mapToMapExpression(definitionMap)]);
   const type = new o.ExpressionType(o.importExpr(R3.NgModuleDefWithMeta, [
     new o.ExpressionType(moduleType), tupleTypeOf(declarations), tupleTypeOf(imports),
     tupleTypeOf(exports)