botserver: Fix mypy error.

[LoopThrough-i-j]

Value of type "Optional[Any]" is not indexable error was originated by mypy.

zulip_botserver/zulip_botserver/server.py:172: error: Value of type "Optional[Any]" is not indexable
zulip_botserver/zulip_botserver/server.py:179: error: Value of type "Optional[Any]" is not indexable
zulip_botserver/zulip_botserver/server.py:180: error: Value of type "Optional[Any]" is not indexable
zulip_botserver/zulip_botserver/server.py:183: error: Value of type "Optional[Any]" is not indexable
zulip_botserver/zulip_botserver/server.py:187: error: Value of type "Optional[Any]" is not indexable
zulip_botserver/zulip_botserver/server.py:188: error: Value of type "Optional[Any]" is not indexable
zulip_botserver/zulip_botserver/server.py:189: error: Value of type "Optional[Any]" is not indexable

and

zulip_botserver/zulip_botserver/server.py:173: error: Value of type "Optional[Any]" is not indexable
zulip_botserver/zulip_botserver/server.py:180: error: Value of type "Optional[Any]" is not indexable
zulip_botserver/zulip_botserver/server.py:181: error: Value of type "Optional[Any]" is not indexable
zulip_botserver/zulip_botserver/server.py:184: error: Value of type "Optional[Any]" is not indexable
zulip_botserver/zulip_botserver/server.py:188: error: Value of type "Optional[Any]" is not indexable
zulip_botserver/zulip_botserver/server.py:189: error: Value of type "Optional[Any]" is not indexable
zulip_botserver/zulip_botserver/server.py:190: error: Value of type "Optional[Any]" is not indexable

was originated in PR #682.

[andersk]

We should be validating that the input is what we expect, rather than checking that the input is not None. (The input could just as easily be a number, a string, an array, a dict without the needed keys, or a dict with the needed keys but with wrong value types.)

[LoopThrough-i-j]

I just considered making a simple fix to remove the error for the subsequent PRs. But we can probably use TypedDict for a better implementation.