fix(pluck): operator breaks with null/undefined inputs. (#5524)

* test(pluck): add failing test case for using null value.

* fix(pluck): check for null/undefined object before attempting to access prop

* Remove null coalescing when checking values to prevent null values being converted to undefined

spec/operators/pluck-spec.ts

@@ -176,4 +176,26 @@ describe('pluck operator', () => {
     expectObservable(r, unsub).toBe(expected);
     expectSubscriptions(a.subscriptions).toBe(asubs);
   });
+
+  it('should support symbols', () => {
+    const sym = Symbol('sym');
+
+    const a =   cold('--x--|', {x: {[sym]: 'abc'}});
+    const asubs =    '^    !';
+    const expected = '--y--|';
+
+    const r = a.pipe(pluck(sym));
+    expectObservable(r).toBe(expected, {y: 'abc'});
+    expectSubscriptions(a.subscriptions).toBe(asubs);
+  });
+
+  it('should not break on null values', () => {
+    const a =   cold('--x--|', {x: null});
+    const asubs =    '^    !';
+    const expected = '--y--|';
+
+    const r = a.pipe(pluck('prop'));
+    expectObservable(r).toBe(expected, {y: undefined});
+    expectSubscriptions(a.subscriptions).toBe(asubs);
+  });
 });

src/internal/operators/pluck.ts

@@ -56,8 +56,8 @@ function plucker(props: string[], length: number): (x: string) => any {
   const mapper = (x: string) => {
     let currentProp = x;
     for (let i = 0; i < length; i++) {
-      const p = currentProp[props[i]];
-      if (typeof p !== 'undefined') {
+      const p = currentProp != null ? currentProp[props[i]] : undefined;
+      if (p !== void 0) {
         currentProp = p;
       } else {
         return undefined;