fix(ngSrc, ngSrcset): only interpolate if all expressions are defined

BREAKING CHANGE

If `bar` is `undefined`, before `<img src="foo/{{bar}}.jpg">` yields
`<img src="foo/.jpg">`. With this change, the binding will not set `src`.

If you want the old behavior, you can do this: `<img src="foo/{{bar || ''}}.jpg">`.

The same applies for `srcset` as well.

Closes angular#6984

src/ng/compile.js

@@ -1854,9 +1854,17 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
                           "ng- versions (such as ng-click instead of onclick) instead.");
                 }
 
+                var allOrNothingAttrs = [
+                  'ngSrc',
+                  'ngSrcset',
+                  'src',
+                  'srcset'
+                ];
+
                 // we need to interpolate again, in case the attribute value has been updated
                 // (e.g. by another directive's compile function)
-                interpolateFn = $interpolate(attr[name], true, getTrustedContext(node, name));
+                interpolateFn = $interpolate(attr[name], true, getTrustedContext(node, name),
+                    allOrNothingAttrs.indexOf(name) > -1);
 
                 // if attribute was updated so that there is no interpolation going on we don't want to
                 // register any observers

test/ng/directive/booleanAttrsSpec.js

@@ -204,7 +204,7 @@ describe('ngSrcset', function() {
     var element = $compile('<div ng-srcset="some/{{id}} 2x"></div>')($rootScope);
 
     $rootScope.$digest();
-    expect(element.attr('srcset')).toEqual('some/ 2x');
+    expect(element.attr('srcset')).toBeUndefined();
 
     $rootScope.$apply(function() {
       $rootScope.id = 1;

test/ng/directive/ngSrcsetSpec.js

@@ -11,6 +11,6 @@ describe('ngSrcset', function() {
     $rootScope.image = {};
     element = $compile('<img ng-srcset="{{image.url}} 2x">')($rootScope);
     $rootScope.$digest();
-    expect(element.attr('srcset')).toEqual(' 2x');
+    expect(element.attr('srcset')).toBeUndefined();
   }));
 });