SslConfiguration should accept blank password for keystore

[subhashreddy22]

SslConfiguration should accept blank password for keystore instead of defaulting password to null when it is blank

https://github.com/spring-cloud/spring-cloud-vault/blob/main/spring-cloud-vault-config/src/main/java/org/springframework/cloud/vault/config/VaultConfiguration.java#L78-L83

The keystores exported from Azure Key Vault have a blank password which at the moment can't be used due to this. https://learn.microsoft.com/en-us/azure/key-vault/certificates/how-to-export-certificate?tabs=azure-cli#export-stored-certificates