return empty result instead of hard-to-understand internal error if imagePaths is of length 0

[czeildi]

If there is no images to annotate and also no saved annotations, function returns error

googleCloudVisionR::gcv_get_image_annotations(character(0))
#> Error in googleCloudVisionR::gcv_get_image_annotations(character(0)): object 'imageAnnotations' not found

^{Created on 2019-08-23 by the reprex package (v0.3.0.9000)}

It might make sense to return a data table with 0 rows (but the same columns you would get otherwise) so that it is easier in a loop. If that is not preferred for some reason I'd welcome a more specific error