Ensure empty files do not have a mimetype

[wagoodman]

Today it seems that non-nil readers to files with no contents are resulting in text/plain MIME types, which is incorrect. We should return no MIME type. This PR makes this adjustment by ensuring the size of read bytes is not 0.

Related to anchore/syft#782

[github-actions]

Benchmark Test Results

Benchmark results from the latest changes vs base branch

name                                old time/op    new time/op    delta
pkg:github.com/anchore/stereoscope/pkg/file goos:linux goarch:amd64
TarIndex-2                            54.2µs ± 1%    58.9µs ± 2%   +8.51%  (p=0.029 n=4+4)
pkg:github.com/anchore/stereoscope/test/integration goos:linux goarch:amd64
SimpleImage_GetImage/FromTarball-2    1.85ms ± 6%    2.04ms ± 4%  +10.15%  (p=0.016 n=5+5)

name                                old alloc/op   new alloc/op   delta
pkg:github.com/anchore/stereoscope/pkg/file goos:linux goarch:amd64
TarIndex-2                            8.38kB ± 0%    8.38kB ± 0%     ~     (p=1.000 n=4+4)
pkg:github.com/anchore/stereoscope/test/integration goos:linux goarch:amd64
SimpleImage_GetImage/FromTarball-2     409kB ± 0%     402kB ± 0%   -1.82%  (p=0.008 n=5+5)

name                                old allocs/op  new allocs/op  delta
pkg:github.com/anchore/stereoscope/pkg/file goos:linux goarch:amd64
TarIndex-2                               205 ± 0%       205 ± 0%     ~     (all equal)
pkg:github.com/anchore/stereoscope/test/integration goos:linux goarch:amd64
SimpleImage_GetImage/FromTarball-2     4.66k ± 0%     4.40k ± 0%   -5.46%  (p=0.029 n=4+4)

[jonasagx]

Maybe add a directory example?

[wagoodman]

since this function takes a reader it's not possible to pass it a directory

[jonasagx]

LGTM with a test suggestion