NullPointerException in AggregateMetric

The getMax and getMin methods in AggregateMetric can throw an NPE if non-Number values are present in values, when it tries to cast a null Double to a double.

This PR prevents the NPE occurring: https://github.com/apache/lucene-solr/pull/1265

(We've also noticed an error in the documentation - see https://github.com/apache/lucene-solr/commit/109d3411cd3866d83273187170dbc5b8b3211d20 - this could be pulled out into a separate ticket if necessary?)