Prefix for secrets manager cannot be empty

[MatejNedic]

Type: Bug

Component:
"Secrets Manager"

Describe the bug
In code and docs
`* Prefix indicating first level for every property. Value must start with a forward

• slash followed by a valid path segment or be empty. Defaults to "/secret".`
  Currently, this is not possible since validation will fail on an empty string.

Sample
Fix Validation for a prefix cannot be the empty string AwsSecretsManagerProperties.class Also change AwsSecretsManagerPropertySources.class method protected String getContext(String prefix, String context) to take changes into account.
Provide tests for all changes.