Fix: Only attempt split_cookie extraction if all of the cookies are present

[carlobeltrame]

Fixes #930

As explained in #930, this isn't a breaking change, because setups with partial cookies couldn't have made sense for anyone before. Some examples:

• 2 cookies jwt_hp and jwt_s, first one missing: Will result in .eySignature, which is not a valid JWT token (note the leading period)
• 2 cookies jwt_hp and jwt_s, second one missing: Will result in eyHeader.eyPayload. which is not a valid JWT token (note the trailing period)
• 2 cookies jwt_complete and optional_suffix, second one missing: Will result in eyHeader.eyPayload.eySignature. which is not a valid JWT token (note the trailing period)
• 3 cookies jwt_h, jwt_p and jwt_s, middle one missing: Will result in eyHeader..eySignature which is not a valid JWT token (note the two consecutive periods)

So up until now, there is no way someone was successfully using the SplitCookieExtractor with only some of the cookies present.

[chalasr]

Good catch, thanks @carlobeltrame.