[UX] Error out for null env var

[Michaelvll]

When an env var is specified in YAML but is empty, sky launch without --env will fail.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
  • sky launch llm/llama-3/llama3.yaml correctly fail
  • sky launch llm/llama-3/llama3.yaml --env HF_TOKEN works.
• 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: conda deactivate; bash -i tests/backward_compatibility_tests.sh

[romilbhardwaj]

Thanks @Michaelvll! Works nicely!