Fix(SCIMMY.Messages.PatchOp): empty array error on custom extension

[sp1r1t]

When running a "Remove" patch operation on an array in a custom extension I get the error:

Cannot read properties of undefined (reading 'length')

The error arises in patchop.js on line 383:

patchop.js:380-383

// Filter out any values that exist in removals list
target[property] = (target[property] ?? []).filter(v => !removals.includes(v));
// Unset the property if it's now empty
if (target[property]).length === 0) target[property] = undefined; // <----

The issue arises because target[property] is a schema instance, which returns undefined when it has no values.

schema.js:204

if (!hasActualValues(resource[extension.id] ?? {})) return undefined;

This PR fixes the issue by safeguarding the .length call by coalescing to an empty array if target[property] is nullish.