Fix slack session when channel id is null

[etrex]

issue

When user clicked a button on home tab, session key can't generate correctly.

spec

If the slack event is not related to any channel, then the session key set to the user id.

[codecov-commenter]

Codecov Report

Merging #802 into release/1.5 will increase coverage by 2.56%. The diff coverage is 86.92%.

@@               Coverage Diff               @@
##           release/1.5     #802      +/-   ##
===============================================
+ Coverage        81.08%   83.65%   +2.56%     
===============================================
  Files              108      115       +7     
  Lines             4747     6068    +1321     
  Branches          1197     1491     +294     
===============================================
+ Hits              3849     5076    +1227     
- Misses             862      988     +126     
+ Partials            36        4      -32     

Impacted Files	Coverage Δ
...s/bottender/src/cli/providers/messenger/profile.ts	83.05% <ø> (-3.56%)	:arrow_down:
packages/bottender/src/console/ConsoleConnector.ts	100.00% <ø> (ø)
packages/bottender/src/console/ConsoleContext.ts	100.00% <ø> (ø)
packages/bottender/src/line/LineEvent.ts	96.64% <ø> (-1.11%)	:arrow_down:
...ckages/bottender/src/messenger/MessengerContext.ts	83.59% <ø> (-1.48%)	:arrow_down:
packages/bottender/src/slack/SlackContext.ts	92.20% <ø> (+6.00%)	:arrow_up:
...ckages/bottender/src/telegram/TelegramConnector.ts	96.93% <ø> (+0.38%)	:arrow_up:
...kages/bottender/src/test-utils/SimulatedContext.ts	87.50% <ø> (-12.50%)	:arrow_down:
packages/bottender/src/types.ts	100.00% <ø> (ø)
packages/bottender/src/viber/ViberConnector.ts	91.56% <ø> (+5.60%)	:arrow_up:
... and 117 more

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