create_blob: allow creation of empty blobs
create_blob: allow creation of empty blobs

A blob with no content is still a valid blob, we should not refuse to
create those. The GitHub API supports them, and so should we.

krxsky/github3.py,itsmemattchung/github3.py,christophelec/github3.py,ueg1990/github3.py,wbrefvem/github3.py,jim-minter/github3.py,degustaf/github3.py,icio/github3.py,balloob/github3.py,agamdua/github3.py,sigmavirus24/github3.py,h4ck3rm1k3/github3.py