Do not crash when completion request context is missing

[OlegSmelov]

Sublime LSP plugin doesn't send the context when doing a textDocument/completion request:

:: --> ruby textDocument/completion(7): {'position': {'line': 10, 'character': 21}, 'textDocument': {'uri': 'file://[redacted]/app.rb'}}

This causes Steep to crash with NoMethodError (undefined method '[]' for nil:NilClass).

I'm not sure if this is a violation of the protocol, but it looks like trigger isn't used for anything important in Steep, so let's handle it gracefully.

[soutaro]

Thanks! The change totally makes sense.

• context is an optional parameter, and Steep should handle the case.
• trigger is not used at all.