Fix compact tc028

[mielvds]

This PR fixes two related tests in the test-suite:

https://w3c.github.io/json-ld-api/tests/compact-manifest.html#tc028 (this also means 100% spec compliance on compaction)

Adding a minimal/empty property-scoped context should not affect the using terms defined in outer context.

https://w3c.github.io/json-ld-api/tests/toRdf-manifest.html#tc036

Adding a minimal/empty property-scoped context should not affect expansion of terms defined in the outer scope.

The issue probably originated from the Javascript port, where empty objects are truthy. Python treats an empty {} as falsey, so @context: {} was stored as False, which later means "reset/nullify the context." Now only explicit null becomes False; empty contexts are preserved.

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
lib/pyld
canon.py	218	4	98%	27, 180, 565–566
context_resolver.py	102	8	92%	70, 129, 143, 189, 195–197, 221
iri_resolver.py	141	3	98%	307–308, 318
jsonld.py	2440	173	93%	274–278, 359, 396, 401, 411, 428–443, 479–480, 522, 530, 550, 558–559, 667–672, 752–753, 768–769, 829–834, 859–860, 875–876, 886–887, 912–913, 954, 964, 970–974, 985–986, 1019, 1028, 1035, 1100, 1123, 1145–1147, 1157–1160, 1175, 1198, 1210–1213, 1305, 1317–1320, 1340, 1369, 1402, 1406, 1443, 1486, 1504–1507, 1516, 1561, 1664–1671, 1698–1700, 1705, 1726–1729, 1905, 1943, 2376, 2385–2392, 2484, 2511, 2534, 2543, 3059, 3112, 3115, 3178, 3263, 3297, 3305, 3583, 3588, 3590, 3639, 3777–3781, 3847, 3884, 3947, 4167–4168, 4182, 4470, 4602, 4636, 4665–4668, 4694–4695, 4878, 4942, 4969, 5043, 5045, 5063, 5161, 5210, 5442, 5444, 5483, 5485, 5494, 5633, 5761, 5848, 5968–5982, 6041–6047, 6114, 6327, 6333–6337, 6376, 6379, 6469–6470, 6479
lib/pyld/documentloader
aiohttp.py	65	22	66%	28–38, 69, 75, 87, 99–107, 118–121, 138, 153–155
requests.py	41	5	88%	55, 69, 81–89
TOTAL	3193	215	93%

Tests	Skipped	Failures	Errors	Time
1673	54 💤	0 ❌	0 🔥	11.636s ⏱️