fix: when filtering workspaces, make sure the edge has a to before checking if its a workspace

[nlf]

WIP: needs tests

when checking if an edge target is a workspace, we should first make sure the edge even has a target. missing optional dependencies can cause a tree that has an edge with no target.

[npm-cli-bot]

no statistically significant performance changes detected

timing results

app-large	clean	lock-only	cache-only	cache-only peer-deps	modules-only	no-lock	no-cache	no-modules	no-clean	no-clean audit
npm@8	40.493 ±0.94	19.870 ±0.16	17.305 ±0.12	21.659 ±3.33	3.264 ±0.28	2.979 ±0.02	2.597 ±0.19	12.010 ±0.01	2.434 ±0.06	4.257 ±0.97
#5164	40.674 ±1.48	19.730 ±0.26	17.780 ±0.21	20.428 ±0.85	3.117 ±0.03	3.058 ±0.00	2.490 ±0.04	12.229 ±0.11	2.510 ±0.00	4.185 ±0.94

app-medium	clean	lock-only	cache-only	cache-only peer-deps	modules-only	no-lock	no-cache	no-modules	no-clean	no-clean audit
npm@8	29.773 ±0.21	14.659 ±0.12	12.957 ±0.06	14.602 ±1.00	2.907 ±0.05	2.919 ±0.05	2.508 ±0.04	8.882 ±0.09	2.320 ±0.05	3.138 ±0.01
#5164	29.595 ±1.07	14.912 ±0.27	12.981 ±0.03	14.358 ±0.40	2.836 ±0.07	2.825 ±0.01	2.491 ±0.07	8.709 ±0.16	2.294 ±0.04	3.204 ±0.02