Force name to be string when looking up mime type

[lildude]

Description

#4531 switched a dependency from using mime-types to mini_mime but missed a scenario that didn't come up until I started testing the forth-coming v.7.5.0 release: _mime_type() will fail if nil is passed as the filename, as is the case when GitHub serves raw data.

This results in MiniMime.lookup_by_filename(name) failing with:

TypeError: no implicit conversion of nil into String
    /var/lib/jenkins/workspace/github/vendor/gems/2.6.2/ruby/2.6.0/gems/mini_mime-1.0.1/lib/mini_mime.rb:45:in `extname'
    /var/lib/jenkins/workspace/github/vendor/gems/2.6.2/ruby/2.6.0/gems/mini_mime-1.0.1/lib/mini_mime.rb:45:in `lookup_by_filename'
    /var/lib/jenkins/workspace/github/vendor/gems/2.6.2/ruby/2.6.0/gems/mini_mime-1.0.1/lib/mini_mime.rb:6:in `lookup_by_filename'
    /var/lib/jenkins/workspace/github/vendor/gems/2.6.2/ruby/2.6.0/gems/github-linguist-7.4.0.32.g7f67bc95/lib/linguist/blob_helper.rb:36:in `_mime_type'
    /var/lib/jenkins/workspace/github/vendor/gems/2.6.2/ruby/2.6.0/gems/github-linguist-7.4.0.32.g7f67bc95/lib/linguist/blob_helper.rb:56:in `binary_mime_type?'
    /var/lib/jenkins/workspace/github/vendor/gems/2.6.2/ruby/2.6.0/gems/github-linguist-7.4.0.32.g7f67bc95/lib/linguist/blob_helper.rb:79:in `content_type'
[...]

This PR fixes issue by forcing the name to always be a string, even if nil.

[ Checklist removed as it doesn't apply ]

/cc @ksolo

[pchaigno]

We don't have tests for this?

[lildude]

We don't have tests for this?

Clearly not 😉. I'll see if I can come up with a good test.

[lildude]

And now with a test 😁

[pchaigno]

LGTM!