fix: change default mongodb replica set to null

[kaustavb12]

(cherry picked from commit 1ad3ecf)

Cherry-pick from master PR openedx-unsupported#6831
This PR changes the default value of EDXAPP_MONGO_REPLICA_SET from empty string ("") to null.

This is required due to unpinnig of pymongo via openedx/edx-platform#30569
As mentioned in that PR, pymongo >= 3.11 only accepts None as replica set if standalone MongoDB instance is used.
Empty string is not accepted as a valid value for replica set.

Other information

Private Ref: BB-6773

Configuration Pull Request

Make sure that the following steps are done before merging

• A devops team member has commented with 👍
• are you adding any new default values that need to be overridden when this goes live?
  • Open a ticket (DEVOPS) to make sure that they have been added to secure vars.
  • Add an entry to the CHANGELOG.
• Are you adding/updating any variables that need to be added/updated to a specific configuration-secure repo?

[navinkarkera]

👍

• I tested this: checked deployment of nutmeg instance
• I read through the code
• I checked for accessibility issues
• Includes documentation
• I made sure any change in configuration variables is reflected in the corresponding client's configuration-secure repository.