Fix type error when saving existing media object

[vhorzinek]

To fix the type error mentioned in this issue:
#1

The problem was:
Since the media object was already existing, and potentially the image file was not changed, payload does not return a files.file object in the request object. Therefore the type error, because getPlaiceholder tried converting nothing to a base64.

To fix the error I, instead of the approach mentioned in the issue, decided to grab the existing image and generae a new buffer which can then be converted to base64 in case there is no base64 value already.

This approach in my opinion has the benefit of not having to reupload every image that might not have a base64 yet, since instead of running into an undefined from a missing upload, it just takes the existing image.