[JENKINS-14504] sloccount churn graph

- Plugin version 1.10 and less stored empty data set to build.xml even for builds that failed. This causes unwanted high peaks in trend with deltas.
- Builds with no data are excluded to fix the issue.
- Indentation fixed.
- Example of legacy build.xml data:

    <hudson.plugins.sloccount.SloccountBuildAction plugin="sloccount@1.10">
      <build class="build" reference="../../.."/>
      <result>
        <report>
          <files class="linked-hash-map"/>
          <lineCount>0</lineCount>
          <folders class="linked-hash-map"/>
          <languages class="linked-hash-map"/>
        </report>
        <owner class="build" reference="../../../.."/>
      </result>
    </hudson.plugins.sloccount.SloccountBuildAction>