[REQUIRED] Environment info

**firebase-tools:**13.15.4

**Platform:**macOS

[REQUIRED] Test case

Use old firebase.json file without extensions field or with:

{
  ... // any other settings
  "extensions": {}
}

[REQUIRED] Steps to reproduce

Just run command: firebase deploy --only functions --force

[REQUIRED] Expected behavior

Deploying only functions.

[REQUIRED] Actual behavior

Deploying only functions and removing all extensions.