fix(clients): use optional chaining while processing runtime config

[gosar]

Related to smithy-lang/smithy-typescript#401

Testing

How was this change tested?

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[gosar]

I think the tests are failing here because smithy-lang/smithy-typescript#401 needs to be merged for it to succeed.

[codecov-commenter]

Codecov Report

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@@           Coverage Diff           @@
##             main    #2690   +/-   ##
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  Coverage        ?   60.81%           
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  Files           ?      531           
  Lines           ?    27890           
  Branches        ?     6843           
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  Hits            ?    16960           
  Misses          ?    10930           
  Partials        ?        0           

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[github-actions]

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