fix(compiler): add an empty content for source file of non mapped code. (#15246)

Before this when using ngc, tools tried to load `ng://…<component>.ts`
if `…<component>.ts` was the source file of a template.

PR Close #15246

packages/core/test/linker/source_map_integration_node_only_spec.ts

@@ -128,7 +128,7 @@ export function main() {
            expect(sourceMap.sources).toEqual([
              'ng:///DynamicTestModule/MyComp.ngfactory.js', ngUrl
            ]);
-           expect(sourceMap.sourcesContent).toEqual([null, template]);
+           expect(sourceMap.sourcesContent).toEqual([' ', template]);
          }));