Allow images without alt attribute

[winniehell]

Current behavior for images with missing alt attribute is:

$ kramdown -ihtml -okramdown <<<'<img src=""/>'
/var/lib/gems/1.9.1/gems/kramdown-1.5.0/lib/kramdown/converter/kramdown.rb:289:in `convert_img': undefined method `gsub' for nil:NilClass (NoMethodError)
    from /var/lib/gems/1.9.1/gems/kramdown-1.5.0/lib/kramdown/converter/kramdown.rb:32:in `convert'
    from /var/lib/gems/1.9.1/gems/kramdown-1.5.0/lib/kramdown/converter/kramdown.rb:59:in `block in inner'
    from /var/lib/gems/1.9.1/gems/kramdown-1.5.0/lib/kramdown/converter/kramdown.rb:52:in `each'
    from /var/lib/gems/1.9.1/gems/kramdown-1.5.0/lib/kramdown/converter/kramdown.rb:52:in `each_with_index'
    from /var/lib/gems/1.9.1/gems/kramdown-1.5.0/lib/kramdown/converter/kramdown.rb:52:in `inner'
    from /var/lib/gems/1.9.1/gems/kramdown-1.5.0/lib/kramdown/converter/kramdown.rb:361:in `convert_root'
    from /var/lib/gems/1.9.1/gems/kramdown-1.5.0/lib/kramdown/converter/kramdown.rb:32:in `convert'
    from /var/lib/gems/1.9.1/gems/kramdown-1.5.0/lib/kramdown/converter/base.rb:104:in `convert'
    from /var/lib/gems/1.9.1/gems/kramdown-1.5.0/lib/kramdown/document.rb:120:in `method_missing'
    from /var/lib/gems/1.9.1/gems/kramdown-1.5.0/bin/kramdown:61:in `block in '
    from /var/lib/gems/1.9.1/gems/kramdown-1.5.0/bin/kramdown:61:in `each'
    from /var/lib/gems/1.9.1/gems/kramdown-1.5.0/bin/kramdown:61:in `'
    from /usr/local/bin/kramdown:23:in `load'
    from /usr/local/bin/kramdown:23:in `'

whereas

$ kramdown -ihtml -okramdown <<<'<img src="" alt=""/>'
![]()

works perfectly fine.

Please consider treating missing alt attributes as empty.

[gettalong]

Thanks for the bug report, this should certainly be done.

[gettalong]

Fixed this.

[winniehell]

Thank you! 👍