message: stats: initialize store_ parameter on mock histograms
  This ensures that the nullptr check in the recordValue default
  handler makes sense, preventing UB when the handler is invoked
  on a default constructed mock histogram.
diff: @@ -209,7 +209,7 @@ public:
  uint32_t use_count() const override { return refcount_helper_.use_count(); }
  Unit unit_{Histogram::Unit::Unspecified};
  - Store* store_;
  + Store* store_{};
  private:
  RefcountHelper refcount_helper_;
  @@ -237,7 +237,7 @@ public:
  bool used_;
  Unit unit_{Histogram::Unit::Unspecified};
  - Store* store_;
  + Store* store_{};
  std::shared_ptr<HistogramStatistics> histogram_stats_ =
  std::make_shared<HistogramStatisticsImpl>();
  