[Config] Fix ssh proxy command to be null

[Michaelvll]

A user reported that the sky launch fails when the following config.yaml is provided:

aws:
      instance_tags:
          mytag: myvalue
      ssh_proxy_command:
          eu-west-1: null
          us-east-1: null
      use_internal_ips: true
      vpc_name: abc

This is due to our recent schema check for config.yaml in #2645

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
  • sky launch -c test --cloud aws
• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh

[concretevitamin]

LGTM. Thanks @Michaelvll.

[iojw]

LGTM! Thank you.