findById returns keys with undefined when fields filter is array

[irfankhan-sf]

Describe the bug
findById returns keys with undefined when fields filter is array. if fields filter is array it return empty object

To Reproduce
Steps to reproduce the behavior:

1. Create a filter using loopback "fields" filter, fields with array of property. For example:-
   fields: [ "id", "header", "content"],
2. when we call findById with this fields filter
3. The object keys with undefined values will returned
   returned values: {id:undefined, header: undefined, content: undefined }

Expected behavior
It should return object with keys that we pass with fields with array

[yeshamavani]

done