Ensure that JSON.GET returns redis.Nil for missing keys/paths,
making it consistent with other Redis commands.

- Restore proper nil detection logic in JSONCmd.readReply
- Add comprehensive test coverage for JSON nil scenarios
- Handle both non-existent keys and non-existent paths consistently
- Distinguish between empty arrays and nil responses
- Add documentation for Val() and Expanded() methods

Original work and problem identification by Nic Gibson.
Enhanced implementation with comprehensive testing and fixes
for the broken nil detection logic.

Fixes #2987

…nil-response

- Non-existent keys return redis.Nil (consistent with other Redis commands)
- Non-existent paths in existing keys return empty array '[]'
- Fix broken test that was using wrong doc1 reference
- Add comprehensive test coverage for JSON nil scenarios

This aligns with official Redis JSON.GET behavior:
- Missing keys should return nil error like other Redis commands
- Missing paths should return empty JSON array, not error