Fix nil return from SlideShare renderer causing NoMethodError

[stuzart]

Summary

• Follow-up to Handle JSON parse errors in SlideShare renderer #2631 (which fixed Potential JSON parse error when rendering slideshare #2629)
• The rescue block in SlideshareRenderer#render_content returned nil on RestClient::Exception or JSON::ParserError, but two call sites call .html_safe directly on the result of render, which would raise NoMethodError if nil is returned
• Changed the rescue to return '' instead, consistent with what BlobRenderer#render itself returns when catching exceptions