fix: empty string check for aws url validation

[sai-sunder-s]

Validate aws metadata urls only if they are not null or empty. It is possible to fetch tokens successfully even if metadata urls are not provided but required environment variables are set. If both metadata urls and env vars are not provided, we will throw an exception.

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Fixes #<issue_number_goes_here> ☕️

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[TimurSadykov]

LGTM, except i would allow uppercase host