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

@@ -196,4 +196,14 @@ describe('pluck operator', () => {
     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

@@ -51,7 +51,7 @@ export function pluck<T, R>(...properties: Array<string | number | symbol>): Ope
   return map((x) => {
     let currentProp: any = x;
     for (let i = 0; i < length; i++) {
-      const p = currentProp[properties[i]];
+      const p = currentProp?.[properties[i]];
       if (typeof p !== 'undefined') {
         currentProp = p;
       } else {