Fixed issue where TabularData would not be returned if the metrics do not have tags

[carlosroman]

This PR fixes an issue where TabularData was not correctly returned if the config had no mappings for tags.

i.e. The following config would work:

      - include:
          domain: org.datadog.jmxfetch.test
          attribute:
            Tabulardata.foo:
              metric_type: gauge
              alias: multiattr.foo
              tags:
                foo: $foo
                toto: $toto
              limit: 1
              sort: desc

But if you remove the tag section (like bellow) it would not return any data back.

      - include:
          domain: org.datadog.jmxfetch.test
          attribute:
            Tabulardata.foo:
              metric_type: gauge
              alias: multiattr.foo
              limit: 1
              sort: desc