Fix "undefined property '__op'" in postgres update (#4541)

* Fix "undefined property '__op'" in postgres update

This causes a TypeError which becomes a regular Error,
before the update can be issued. (I think)

This happens when there is an object schema,
and there is also an unrelated field in originalUpdate
which is null or undefined.

e.g. when 'location' is a mandatory object in postgres,
and 'middleName' is an optional string,
PostgresStorageAdapter would throw when a query similar
to the below was performed:
(Object.keys(originalUpdate) would include "middleName" as a value of `k`)

    query.set('location', {'country': 'US'})
    query.set('middleName', undefined);

* Fix lint error

src/Adapters/Storage/Postgres/PostgresStorageAdapter.js

@@ -1200,7 +1200,11 @@ export class PostgresStorageAdapter implements StorageAdapter {
         // Gather keys to increment
         const keysToIncrement = Object.keys(originalUpdate).filter(k => {
           // choose top level fields that have a delete operation set
-          return originalUpdate[k].__op === 'Increment' && k.split('.').length === 2 && k.split(".")[0] === fieldName;
+          // Note that Object.keys is iterating over the **original** update object
+          // and that some of the keys of the original update could be null or undefined:
+          // (See the above check `if (fieldValue === null || typeof fieldValue == "undefined")`)
+          const value = originalUpdate[k];
+          return value && value.__op === 'Increment' && k.split('.').length === 2 && k.split(".")[0] === fieldName;
         }).map(k => k.split('.')[1]);
 
         let incrementPatterns = '';
@@ -1216,8 +1220,9 @@ export class PostgresStorageAdapter implements StorageAdapter {
         }
 
         const keysToDelete = Object.keys(originalUpdate).filter(k => {
-          // choose top level fields that have a delete operation set
-          return originalUpdate[k].__op === 'Delete' && k.split('.').length === 2 && k.split(".")[0] === fieldName;
+          // choose top level fields that have a delete operation set.
+          const value = originalUpdate[k];
+          return value && value.__op === 'Delete' && k.split('.').length === 2 && k.split(".")[0] === fieldName;
         }).map(k => k.split('.')[1]);
 
         const deletePatterns = keysToDelete.reduce((p, c, i) => {