fix(compiler): detect and report error for views with empty templateUrl

Fixes #3762

Closes #3768

modules/angular2/src/core/compiler/compiler.ts

@@ -301,7 +301,7 @@ export class Compiler {
         this._urlResolver.resolve(this._appUrl, this._componentUrlMapper.getUrl(component));
     var templateAbsUrl = null;
     var styleAbsUrls = null;
-    if (isPresent(view.templateUrl)) {
+    if (isPresent(view.templateUrl) && view.templateUrl.trim().length > 0) {
       templateAbsUrl = this._urlResolver.resolve(componentUrl, view.templateUrl);
     } else if (isPresent(view.template)) {
       // Note: If we have an inline template, we also need to send

modules/angular2/test/core/compiler/compiler_spec.ts

@@ -149,6 +149,26 @@ export function main() {
                });
          }));
 
+      it('should not fill templateAbsUrl given template url with empty string',
+         inject([AsyncTestCompleter], (async) => {
+           cmpUrlMapper.setComponentUrl(MainComponent, '/cmp/main.js');
+           captureTemplate(new ViewMetadata({template: null, templateUrl: ''}))
+               .then((renderTpl) => {
+                 expect(renderTpl.templateAbsUrl).toBe(null);
+                 async.done();
+               });
+         }));
+
+      it('should not fill templateAbsUrl given template url with blank string',
+         inject([AsyncTestCompleter], (async) => {
+           cmpUrlMapper.setComponentUrl(MainComponent, '/cmp/main.js');
+           captureTemplate(new ViewMetadata({template: null, templateUrl: '   '}))
+               .then((renderTpl) => {
+                 expect(renderTpl.templateAbsUrl).toBe(null);
+                 async.done();
+               });
+         }));
+
       it('should fill templateAbsUrl given url template', inject([AsyncTestCompleter], (async) => {
            cmpUrlMapper.setComponentUrl(MainComponent, '/cmp/main.js');
            captureTemplate(new ViewMetadata({templateUrl: 'tpl/main.html'}))