Add tests against multiple MongoDB versions (#7161)

* added tests environment with mongodb 4.4.3

* added CI test for mongodb 4.4.3

* added CI tests for MongoDB versions 4.0, 4.2

* improved flaky test (seems to max out the limit of simultaneous connections)

* added spec helpers to run tests only for specific MongoDB version

* addedn npm scripts to run tests against relevant mongodb versions

* added spec helper function to exclude specific mongodb version

* added test for changed aggregate query planner results

* fixed regex test with incorrect regex syntax

* fixed test where query has select no keys (empty array)

* added changelog entry and ordered list

* fixed test that tried to simultaneously delete and build index on same collection

* added MongoDB compatibility table to readme

* updated default local tests to use MongoDB 4.4.3

* added MongoDB badges for new versions to README

* fixed typo in readme

* added new test helper filter to contribution guide

* fixed incorrect storage engine for mongodb 4.4

* changed CI to test MongoDB 3.6. with mmapv1 storage engine and standalone

* improved CI test description

* added CI self check for new MongoDB versions

* fixed CI

* removed CI

* added CI

* added throwing error if any of the checks failed

* added github action connector

* improved error message

* improved error messages

* improved error message

* updated CI environment to MongoDB 3.6.22

* improved error messages

* update CI env name

* updated CI env name

* improved error message

* removed patch versions from CI env description

* improved status message

* removed version range from core lib

* added explicit mongodb version to redis test and node 12 test

* bumped Node 12 test to 12.20.1 (version currently recommended by AWS Elastic Beanstalk)

src/RestQuery.js

@@ -100,7 +100,7 @@ function RestQuery(
   for (var option in restOptions) {
     switch (option) {
       case 'keys': {
-        const keys = restOptions.keys.split(',').concat(AlwaysSelectedKeys);
+        const keys = restOptions.keys.split(',').filter(key => key.length > 0).concat(AlwaysSelectedKeys);
         this.keys = Array.from(new Set(keys));
         break;
       }