fix: add a placeholder changeset if diffing fails

[ltucker]

improves information available when an ingestion log that failed diffing is presented as a deviation

if diffing fails, an empty changeset with an appropriate deviation name and branch are created.

[github-actions]

Go test coverage

STATUS	ELAPSED	PACKAGE	COVER	PASS	FAIL	SKIP
🟢 PASS	0.01s	github.com/netboxlabs/diode/diode-server/dbstore/postgres	0.0%	0	0	0
🟢 PASS	1.22s	github.com/netboxlabs/diode/diode-server/ingester	75.4%	9	0	0
🟢 PASS	1.06s	github.com/netboxlabs/diode/diode-server/migrator	70.4%	4	0	0
🟢 PASS	0.01s	github.com/netboxlabs/diode/diode-server/netbox	0.0%	0	0	0
🟢 PASS	1.71s	github.com/netboxlabs/diode/diode-server/netboxdiodeplugin	78.9%	42	0	0
🟢 PASS	1.42s	github.com/netboxlabs/diode/diode-server/reconciler	77.1%	43	0	0
🟢 PASS	1.09s	github.com/netboxlabs/diode/diode-server/reconciler/applier	81.8%	1	0	0
🟢 PASS	0.01s	github.com/netboxlabs/diode/diode-server/reconciler/changeset	0.0%	0	0	0
🟢 PASS	1.34s	github.com/netboxlabs/diode/diode-server/reconciler/differ	74.2%	92	0	0
🟢 PASS	1.02s	github.com/netboxlabs/diode/diode-server/server	85.7%	14	0	0
🟢 PASS	1.01s	github.com/netboxlabs/diode/diode-server/version	100.0%	1	0	0

Total coverage: 71.7%