Fix LSP workers crash on file removal

[tk0miya]

When a file is removed, VSCode sends a $/file/reset request where the content field is nil.

For example, the following request is sent from VSCode:

[Steep 1.7.1] [interaction:interaction] [frontend] request={:method=>"$/file/reset", :params=>{:uri=>"file:///Users/tkomiya/work/tmp/tmp/sig/app.rbs", :content=>nil}, :jsonrpc=>"2.0"}

This update adds fallbacks to an empty string when the content is nil.

Closes #1236