Without the if it was possible that images[vendor] was
undefined because their was no image available for that
vendor. And then `Object.entries(images[vendor])` resulted in:

Uncaught TypeError: Cannot convert undefined or null to object

And this led to the problem that no device at all was shown :(

Fixes d45019d1 ("app: filter vendors without images for enabled device categories")
Fixes #118
Closes #122