fix(ngModel): treat synchronous validators as boolean always

Change synchronous validators to convert the return to boolean value.
Prevent unexpected behavior when returning `undefined`.

Closes #14734
Closes #15208

BREAKING CHANGE: Previously, only a literal `false` return would resolve as the
synchronous validator failing. Now, all traditionally false JavaScript values
are treated as failing the validator, as one would naturally expect.

Specifically, the values `0` (the number zero), `null`, `NaN` and `''` (the
empty string) used to considered valid (passing) and they are now considered
invalid (failing). The value `undefined` was treated similarly to a pending
asynchronous validator, causing the validation to be pending. `undefined` is
also now considered invalid.

To migrate, make sure your synchronous validators are returning either a
literal `true` or a literal `false` value. For most code, we expect this to
already be the case. Only a very small subset of projects will be affected.

Namely, anyone using `undefined` or any falsy value as a return will now see
their validation failing, whereas previously falsy values other than `undefined`
would have been seen as passing and `undefined` would have been seen as pending.

src/ng/directive/ngModel.js

@@ -603,7 +603,7 @@ NgModelController.prototype = {
     function processSyncValidators() {
       var syncValidatorsValid = true;
       forEach(that.$validators, function(validator, name) {
-        var result = validator(modelValue, viewValue);
+        var result = Boolean(validator(modelValue, viewValue));
         syncValidatorsValid = syncValidatorsValid && result;
         setValidity(name, result);
       });

test/ng/directive/ngModelSpec.js

@@ -847,6 +847,32 @@ describe('ngModel', function() {
         expect(ctrl.$valid).toBe(true);
       });
 
+      it('should treat all responses as boolean for synchronous validators', function() {
+        var expectValid = function(value, expected) {
+          ctrl.$modelValue = undefined;
+          ctrl.$validators.a = valueFn(value);
+
+          ctrl.$validate();
+          expect(ctrl.$valid).toBe(expected);
+        };
+
+        // False tests
+        expectValid(false, false);
+        expectValid(undefined, false);
+        expectValid(null, false);
+        expectValid(0, false);
+        expectValid(NaN, false);
+        expectValid('', false);
+
+        // True tests
+        expectValid(true, true);
+        expectValid(1, true);
+        expectValid('0', true);
+        expectValid('false', true);
+        expectValid([], true);
+        expectValid({}, true);
+      });
+
 
       it('should register invalid validations on the $error object', function() {
         var curry = function(v) {