fix($templateRequest): trust empty templates in $templateCache as well

Implicitly trust empty templates added to `$templateCache` as is the case for all other templates.

Fixes #14479

Closes #14496

Author: rjamet

src/ng/templateRequest.js

@@ -68,7 +68,7 @@ function $TemplateRequestProvider() {
       // are included in there. This also makes Angular accept any script
       // directive, no matter its name. However, we still need to unwrap trusted
       // types.
-      if (!isString(tpl) || !$templateCache.get(tpl)) {
+      if (!isString(tpl) || isUndefined($templateCache.get(tpl))) {
         tpl = $sce.getTrustedResourceUrl(tpl);
       }
 

test/ng/templateRequestSpec.js

@@ -158,6 +158,39 @@ describe('$templateRequest', function() {
     }).not.toThrow();
   }));
 
+  it('should accept empty templates and refuse null or undefined templates in cache',
+    inject(function($rootScope, $templateRequest, $templateCache, $sce) {
+
+    // Will throw on any template not in cache.
+    spyOn($sce, 'getTrustedResourceUrl').and.returnValue(false);
+
+    expect(function() {
+      $templateRequest('tpl.html'); // should go through $sce
+      $rootScope.$digest();
+    }).toThrow();
+
+    $templateCache.put('tpl.html'); // is a no-op, so $sce check as well.
+    expect(function() {
+      $templateRequest('tpl.html');
+      $rootScope.$digest();
+    }).toThrow();
+    $templateCache.removeAll();
+
+    $templateCache.put('tpl.html', null); // makes no sense, but it's been added, so trust it.
+    expect(function() {
+      $templateRequest('tpl.html');
+      $rootScope.$digest();
+    }).not.toThrow();
+    $templateCache.removeAll();
+
+    $templateCache.put('tpl.html', ''); // should work (empty template)
+    expect(function() {
+      $templateRequest('tpl.html');
+      $rootScope.$digest();
+    }).not.toThrow();
+    $templateCache.removeAll();
+  }));
+
   it('should keep track of how many requests are going on',
     inject(function($rootScope, $templateRequest, $httpBackend) {