client: never create clients with nil cookie jars

[bradtgmurray]

With #51, we worked around a crash at startup by handling our cookie jar being nil, but that just caused explosions down the line when we tried to do other operations such as logging out which would attempt to use our cookies (c.getCSRFToken refers to c.jar which is nil).

linkedin/pkg/linkedingo/self.go

Lines 57 to 62 in b4fddae

	func (c *Client) Logout(ctx context.Context) error {
	_, err := c.newAuthedRequest(http.MethodGet, linkedInLogoutURL).
	WithQueryParam("csrfToken", c.getCSRFToken()).
	Do(ctx, nil)
	return err
	}

Just make sure our linkedingo.Client is never created with nil cookies. This also makes the previous change unecessary. Pushed IsLoggedIn down to the linkedingo client to make things more similar to our other bridges instead of checking the origina userLogin struct. Also, skip logging out if we're not logged in, there's no credentials available to make the logout request with.