fix: Add safety check for b:ycm_hover

[ZaharChernenko]

PR Prelude

• I have read and understood YCM's [CONTRIBUTING][cont] document.
• I have read and understood YCM's [CODE_OF_CONDUCT][code] document.
• I have included tests for the changes in my PR. If not, I have included a
  rationale for why I haven't.
• I understand my PR may be closed if it becomes obvious I didn't
  actually perform all of these steps.

Why this change is necessary and useful

The function s:ShowHoverResult() assumes b:ycm_hover always exists when processing hover responses. However, this assumption is unsafe, and leads to errors like this:

Error detected while processing function <SNR>151_PollCommands[30]..<SNR>151_ShowHoverResult:
line   36:
E121: Undefined variable: b:ycm_hover
Error detected while processing function <SNR>151_PollCommands[30]..<SNR>151_ShowHoverResult:
line   36:
E116: Invalid arguments for function has_key( b:ycm_hover, 'popup_params' )
Error detected while processing function <SNR>151_PollCommands[30]..<SNR>151_ShowHoverResult:
line   42:
E121: Undefined variable: b:ycm_hover
Error detected while processing function <SNR>151_PollCommands[30]..<SNR>151_ShowHoverResult:
line   42:
E116: Invalid arguments for function setbufvar

So I added a check for the existence of a variable.

---

This change is