Diff errors when commits include removed submodule

[scott-fleischman]

The following returns an error:

GitHub.Commits.diff "scott-fleischman" "repo-remove-submodule" "d03c152482169d809be9b1eab71dcf64d7405f76" "42cfd732b20cd093534f246e630b309186eb485d"

Left (ParseError "Error in $[2]['blob_url']: failed to parse field files: expected URL, encountered Null")

You can see the example here with the response JSON:
https://github.com/scott-fleischman/repo-remove-submodule

Would you be ok with a change like the following? If so I can put together a PR.

data File = File
    { fileBlobUrl   :: !(Maybe URL)
    , fileStatus    :: !Text
    , fileRawUrl    :: !(Maybe URL)
    , fileAdditions :: !Int

[phadej]

Please, make a PR!