[runtime env] [Serve] Fix error when uris field is None

[architkulkarni]

Why are these changes needed?

When Ray Serve starts a deployment, it overwrites the uris field with the uris from the JobConfig. However, if there are no uris, it sets runtime_env[uris] to None, which causes an error later on when we try to iterate through runtime_env[uris]. This PR handles the None case and adds a test.

Related issue number

Closes #18865

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[architkulkarni]

mac-small-and-large timed out during basic tests, and wheels-and-jars failed even though the post-wheel-build tests succeeded (which implies the wheels successfully built there.) Retrying these tests, let's see what happens