Fix async conn for none aws_session_token

[pankajastro]

currently, if I'm using AWS async conn without aws_session_token in my extra it fails with below error

    async with hook.async_conn as client:
  File "/opt/***/***/providers/amazon/aws/hooks/base_aws.py", line 659, in async_conn
    return self.get_client_type(region_name=self.region_name, deferrable=True)
  File "/opt/***/***/providers/amazon/aws/hooks/base_aws.py", line 605, in get_client_type
    session = self.get_session(region_name=region_name, deferrable=deferrable)
  File "/opt/***/***/providers/amazon/aws/hooks/base_aws.py", line 578, in get_session
    ).create_session(deferrable=deferrable)
  File "/opt/***/***/providers/amazon/aws/hooks/base_aws.py", line 171, in create_session
    self._apply_session_kwargs(session)
  File "/opt/***/***/providers/amazon/aws/hooks/base_aws.py", line 143, in _apply_session_kwargs
    self.conn.session_kwargs["aws_session_token"],

here, I'm changing to get aws_session_token value using get on dic and not indexing

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[o-nikolas]

@syedahsn

[syedahsn]

Nice work! Looks good to me