provider/aws: Fix crash in ElastiCache param group

[catsby]

Fixes crash reported in #6466

When you edit/save the defaults on the web UI (even without changing anything), you get a lot of parameters with default values added. One such parameter (notify-keyspace-events) is returned that has no value 😱

{
  DataType: "string",
  Description: "The keyspace events for Redis to notify Pub/Sub clients about. By default all notifications are disabled",
  IsModifiable: true,
  MinimumEngineVersion: "2.8.6",
  ParameterName: "notify-keyspace-events",
  Source: "user"
},

This PR only sets parameters locally if there is a value found

[jen20]

LGTM!

[ghost]

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