[C++][Parquet] FileKeyWrapper uses KMSConnectionConfig without setting default values

[adamreeve]

See this comment for context: #51350 (comment)

SetDefaultIfEmpty is called on the kms_connection_config_ member, but then the original, unmodified config is passed to GetKmsClient.

This means that the KMS URL and ID can end up being empty when passed to the KmsClientFactory. In the read paths this is not the case, the defaults are used to replace empty values before creating the client factory.