[7.x] Fix an NPE in TransportPutRollupJobAction

[joegallo]

Note: This issue only affects 7.x -- master already works as intended.

We return an error either way, but this way we're returning the intended "you can't do that and here's why" error rather than something the user wouldn't grok.

Kinda related to #32965, but I think this might go all the way back to when rollups were first introduced.

Reproduction

PUT foo
{
  "mappings": {
    "properties": {
      "the_field": { "type": "date" },
     "value_field": { "type": "integer" }
    }
  }
}

PUT _rollup/job/foo-1
{
  "index_pattern": "foo",
  "rollup_index": "non-existent-index",
  "cron": "*/30 * * * * ?",
  "page_size" :10,
  "groups" : {
    "date_histogram": {
      "field": "the_field",
      "calendar_interval": "1h"
    }
  },
  "metrics": [
    {
      "field": "value_field",
      "metrics": ["min", "max", "sum"]
    }
  ]
}

PUT non-rollup

PUT _rollup/job/foo-2
{
  "index_pattern": "foo",
  "rollup_index": "non-rollup",
  "cron": "*/30 * * * * ?",
  "page_size" :10,
  "groups" : {
    "date_histogram": {
      "field": "the_field",
      "calendar_interval": "1h"
    }
  },
  "metrics": [
    {
      "field": "value_field",
      "metrics": ["min", "max", "sum"]
    }
  ]
}

7.x error without this change

{
  "error" : {
  "root_cause" : [
      {
        "type" : "runtime_exception",
        "reason" : "Could not update mappings for rollup job [foo-2]"
      }
    ],
    "type" : "runtime_exception",
    "reason" : "Could not update mappings for rollup job [foo-2]",
    "caused_by" : {
      "type" : "null_pointer_exception",
      "reason" : "Cannot invoke \"org.elasticsearch.cluster.metadata.MappingMetadata.getSourceAsMap()\" because \"mappings\" is null"
    }
  },
  "status" : 500
}

master error without this change, 7.x error with this change

{
  "error" : {
    "root_cause" : [
      {
        "type" : "runtime_exception",
        "reason" : "Rollup data cannot be added to existing indices that contain non-rollup data (expected to find _meta key in mapping of rollup index [non-rollup] but not found)."
      }
    ],
    "type" : "runtime_exception",
    "reason" : "Rollup data cannot be added to existing indices that contain non-rollup data (expected to find _meta key in mapping of rollup index [non-rollup] but not found)."
  },
  "status" : 500
}

[elasticmachine]

Pinging @elastic/es-analytics-geo (Team:Analytics)

[csoulios]

LGTM!