Hover is broken for types that don't have a doc comment

[srivatsn]

Environment data

This is using the latest sources from master

Steps to reproduce

Hover over a normal type in source that doesn't have a doc comment

Expected behavior

Should show a hover tip with the name of the type

Actual behavior

Shows nothing.

Looks like there's a missing null check for the structured doc comment.

/cc @rchande

[rchande]

@akshita31 Can you take a look? It looks like https://github.com/OmniSharp/omnisharp-vscode/blob/master/src/features/documentation.ts#L39 needs to handle a null structured documentation getting passed in.