readers: allow id of -1 in JGF

[jameshcorbett]

Problem: if a vertex's ID is -1 in JGF, the reader will supply it with a default value. However, sometimes -1 is desired.

Do not replace an ID of -1 with a default value when reading in JGF. Instead use -2 as the signpost that no ID was supplied.

Fixes #1303

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 75.3%. Comparing base (cc2dcf0) to head (d03405f). Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1304 +/- ## ====================================== Coverage 75.3% 75.3% ====================================== Files 111 111 Lines 15300 15300 ====================================== Hits 11531 11531 Misses 3769 3769 ``` | [Files with missing lines](https://app.codecov.io/gh/flux-framework/flux-sched/pull/1304?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flux-framework) | Coverage Δ | | |---|---|---| | [resource/readers/resource\_reader\_jgf.cpp](https://app.codecov.io/gh/flux-framework/flux-sched/pull/1304?src=pr&el=tree&filepath=resource%2Freaders%2Fresource_reader_jgf.cpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flux-framework#diff-cmVzb3VyY2UvcmVhZGVycy9yZXNvdXJjZV9yZWFkZXJfamdmLmNwcA==) | `70.2% <100.0%> (ø)` | |