Safely access index field

[nmuesch]

The newly added indexes field to the log_stream widget schema was being directly accessed, this PR first checks if it exists, adding a safety layer to prevent panics.

Fixes #534

I was able to reproduce this by creating a dashboard with a logstream widget using the provider version 2.7.0 like so:

  title         = "Test Log Stream Dashboard"
  description   = "Created using the Datadog provider in Terraform"
  layout_type   = "free"
  is_read_only  = false

  widget {
    log_stream_definition {
      logset = "19"
      query = "error"
      columns = ["core_host", "core_service", "tag_source"]
    }
    layout = {
      height = 36
      width = 32
      x = 5
      y = 51
    }
  }
}

which leads to a terraform state not having indexes set. Then upgrading to 2.8.0 and running a terraform apply, which was trying to directly access this field and throwing an error on refreshing/reading existing resources.