fix(ngStyle): correctly remove old style when new style value is invalid

Since d6098eeb1, old styles were not removed if `newStyles` specified an
invalid value for the style (e.g. `false`). The assumption was that the
new style would overwrite the old style value, but using an invalid
value made browsers ignore the new value and thus keep the old style.
This would typically happen when guarding a style with a boolean flag;
e.g.: `ng-style="{backgroundColor: isError && 'red'}"`

This commit essentially revers commit d6098eeb1, whose main purpose was
to work around jquery/jquery#4185. The jQuery issue has been fixed in
3.4.0, so that should not be a problem any more.

Fixes #16860

Closes #16868

src/ng/directive/ngStyle.js

@@ -54,14 +54,7 @@
 var ngStyleDirective = ngDirective(function(scope, element, attr) {
   scope.$watchCollection(attr.ngStyle, function ngStyleWatchAction(newStyles, oldStyles) {
     if (oldStyles && (newStyles !== oldStyles)) {
-      if (!newStyles) {
-        newStyles = {};
-      }
-      forEach(oldStyles, function(val, style) {
-        if (newStyles[style] == null) {
-          newStyles[style] = '';
-        }
-      });
+      forEach(oldStyles, function(val, style) { element.css(style, ''); });
     }
     if (newStyles) element.css(newStyles);
   });

test/ng/directive/ngStyleSpec.js

@@ -143,6 +143,17 @@ describe('ngStyle', function() {
       expect(element.css(postCompStyle)).not.toBe('99px');
     });
 
+    it('should clear style when the value is false', function() {
+      scope.styleObj = {'height': '99px', 'width': '88px'};
+      scope.$apply();
+      expect(element.css(preCompStyle)).toBe('88px');
+      expect(element.css(postCompStyle)).toBe('99px');
+      scope.styleObj = {'height': false, 'width': false};
+      scope.$apply();
+      expect(element.css(preCompStyle)).not.toBe('88px');
+      expect(element.css(postCompStyle)).not.toBe('99px');
+    });
+
     it('should set style when the value is zero', function() {
       scope.styleObj = {'height': '99px', 'width': '88px'};
       scope.$apply();