fix: create returns undefined if data is empty array (for multi)

[bwgjoseph]

Currently, if the service is defined as multi = ['create'] and pass in empty array to service.create, it will throw TypeError: Cannot read property 'map' of undefined

Hence, this fix adds check to return empty array if the result is undefined so it exits early, and does not throw error

Also, because the test runs on @feathersjs/adapter-tests, I can add an updated test case to .create multi if required

const emptyData = await service.create([]);

assert.ok(Array.isArray(emptyData));
assert.ok(emptyData.length === 0);

If this does gets accepted, do consider adding topic and label for hacktoberfest so it counts towards the contribution