MINIFICPP-1556 Fix setting partial or invalid credentials in AWS credentials service

[lordgamez]

The issue addressed is when partial credentials were set for AWS in the credentials service it could happen that the credential files property was set with the value of a previous property.

Another issue addressed is if the credentials in the credentials service were empty or invalid they were used in the S3 processors. Now instead the S3 processors fail before trying to use the invalid credentials.

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[fgerlits]

LGTM