fix get aws creds from environment

[famarting]

Description

fixes a bug introduced here https://github.com/dapr/components-contrib/pull/3591/files#diff-017233cbc5865ed02ab3eacc4bc99b36ced96fcaf3420176e41d414444e41bc9

previously it was possible to get an aws session without providing access key and secret key, and we would pick the credentials from the environment. But since that PR, specifically because of this

func newStaticIAM(_ context.Context, opts Options, cfg *aws.Config) (*StaticAuth, error) {
	auth := &StaticAuth{
		logger:        opts.Logger,
		region:        &opts.Region,
		endpoint:      &opts.Endpoint,
		accessKey:     &opts.AccessKey,
		secretKey:     &opts.SecretKey,
		sessionToken:  &opts.SessionToken,
		assumeRoleARN: &opts.AssumeRoleARN,
		sessionName:   &opts.SessionName,

and the later check

if a.accessKey != nil && a.secretKey != nil {
		// session token is an option field
		awsConfig = awsConfig.WithCredentials(credentials.NewStaticCredentials(*a.accessKey, *a.secretKey, *a.sessionToken))
	}

that would always think that access key and secret key are available, despite of them having a value or not

this can cause this error from aws

EmptyStaticCreds: static credentials are empty 0xc0003f8600

Issue reference

We strive to have all PR being opened based on an issue, where the problem or feature have been discussed prior to implementation.

Please reference the issue this PR will close: #[issue number]

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Extended the documentation / Created issue in the https://github.com/dapr/docs/ repo: dapr/docs#[issue number]

[famarting]

ready again @elena-kolevska