Only use accessKey and secretKey if they are not blank - fix NullPointerException: Access key ID cannot be blank

[samholton]

Only use the accessKey and secretKey if they are provided. Before it was only checking if role was blank. This allows the creation of a secret that uses the instance profile. This is useful if you want to parameterize the secret being passed in - one for the instance profile and a different one for a different account. This functionality existed before in earlier versions of the plugin, this is restoring that functionality.

Fixes #264

Testing done

• Installed latest version 240.v6d844a_6f5480
• Created two credentials:
  • one named withKey that had a valid access and secret access key (from a second account) and blank role
  • one named withBlanks that had a blank access key, blank secret access key, and blank role
• Ran a job that called aws s3 ls wrapped in withCredentials with both credentials
• The one using withKey worked and listed buckets in the second account
• The one using withBlanks failed with NullPointerException
• Installed the plugin version from the PR and reran the job
• This time both blocks printed out buckets, one from the second account and one from the account the Jenkins node was running within

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