fix(on-call): handle empty 200 body in pages get

[platinummonkey]

What does this PR do?

Fixes pup on-call pages get <id> failing with Error: failed to get page: EOF while parsing value at line 1 column 0.

The Datadog GET /api/v2/on-call/pages/{id} endpoint can return HTTP 200 with an empty body (content-length: 0). The shared parse_response_json helper in src/raw_client.rs passed those empty bytes straight to serde_json, which errored on the missing JSON value. That helper backs raw_get, raw_post, raw_put, and every other hand-written raw HTTP call, so the crash could hit any endpoint returning an empty success body.

An empty or whitespace-only success body is now treated as JSON null, mirroring the existing 204 No Content handling already present in raw_put and raw_request.

Motivation

Reported in issue #638: pup on-call pages get never returns the page and instead errors out. The reporter confirmed via pup api -i v2/on-call/pages/<id> that the endpoint responds 200 with content-length: 0, so the body is genuinely empty and the client must not treat that as a parse failure.

Changes

• src/raw_client.rs: guard empty/whitespace-only bodies in parse_response_json, returning serde_json::Value::Null instead of failing to parse.
• src/raw_client.rs: add raw_get unit tests for an empty body, a whitespace-only body, and a valid JSON body (the last ensures the new guard does not shadow normal parsing).
• src/commands/on_call.rs: add regression test test_on_call_pages_get_empty_body for pages_get receiving an empty 200 body.

Testing

• Positive tests: empty body -> null and whitespace-only body -> null (src/raw_client.rs).
• Negative/contrast test: a valid JSON body still parses correctly (src/raw_client.rs).
• Regression test reproducing issue [BUG] on-call pages get returning EOF while parsing value at line 1 column 0 #638: test_on_call_pages_get_empty_body (src/commands/on_call.rs).
• Existing pages_get tests (success, 404 not found, percent-encoded id) continue to cover the non-empty and error paths.
• cargo fmt --check passes. cargo test / cargo clippy could not be executed in this sandbox because the pinned datadog-api-client git dependency is outside the environment's network allowlist; CI will exercise them.

Additional Notes

The fix is centralized in parse_response_json rather than special-cased in the on-call command, so other raw endpoints that return empty success bodies benefit from the same graceful handling.

Checklist

• The code change follows the project conventions (see CONTRIBUTING.md)
• Tests have been added/updated (if applicable)
• Documentation has been updated (if applicable)
• All CI checks pass
• Code coverage is maintained or improved

Related Issues

Closes #638

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