HPCC-12705 Check before set measure in StatisticsFilter::setKind()

The existing StatisticsFilter::setKind(const char * _kind) sets
the measure to SMeasureAll even if the measure has been set (for
example to 'ns') in StatisticsFilter::set(). In this fix, an if
statement is added before set the measure in the setKind().

Signed-off-by: wangkx <kevin.wang@lexisnexis.com>