replication_changes_left should not be zero when sample is missing per replication

When sample is missing for changes_left per replication, aggregated stat "replication_changes_left" returns 0, instead it should returns a value indicate the sample is missing.

{1428293400000,
{stat_entry,1428293400000,
[

{<<"replication_changes_left">>,0.0}

,

{<<"replication_docs_rep_queue">>,0.0}]}}
{1428294000000,
{stat_entry,1428294000000,
[{<<"replication_changes_left">>,26786478.75},
{<<"replication_docs_rep_queue">>,0.0}

,

{<<"replications/78f09b6d6c59c6971c1ab2cefecb382e/bucket-1/bucket-1/bandwidth_usage">>, 0.0}

,

{<<"replications/78f09b6d6c59c6971c1ab2cefecb382e/bucket-1/bucket-1/changes_left">>, 26786478.75}

,