#14766 Fix potentially uninitialised object used for accumulation

RigStatisticsCalculator::mobileVolumeWeightedMean() accumulated into a sum
from an out-parameter that callees were not required to write. Every
implementation of the per time step overload has early-return paths that
leave the value untouched, as does RigWeightedMeanCalc::weightedMeanOverCells,
so indeterminate stack memory could be summed and presented in the UI.

Return std::optional<double> from the computation layer instead of using an
out-parameter, making "value not available" explicit and impossible to ignore:

- RigWeightedMeanCalc::weightedMeanOverCells
- RigStatisticsCalculator::mobileVolumeWeightedMean and all overrides

Time steps without a computable mean no longer contribute to the accumulated
sum, and are excluded from the divisor. Previously they would either add
garbage or, with a naive zero initialisation, bias the average towards zero.

RigStatisticsDataCache keeps the existing HUGE_VAL marker on its public API
via value_or(), so behaviour seen by RigHistogramData and the overlay info
is unchanged apart from the fix.

Also:
- Set m_isVolumeWeightedMeanCalculated in the per time step cache, which was
  never assigned, so the cached value was recomputed on every call.
- Remove a duplicated ensureKnownResultLoaded() call in
  RigEclipseNativeVisibleCellsStatCalc.

Fixes #14766

ApplicationLibCode/ReservoirDataModel/RigFlowDiagStatCalc.cpp

@@ -115,10 +115,10 @@ size_t RigFlowDiagStatCalc::timeStepCount()
 //--------------------------------------------------------------------------------------------------
 ///
 //--------------------------------------------------------------------------------------------------
-void RigFlowDiagStatCalc::mobileVolumeWeightedMean( size_t timeStepIndex, double& mean )
+std::optional<double> RigFlowDiagStatCalc::mobileVolumeWeightedMean( size_t timeStepIndex )
 {
     auto eclCase = m_resultsData->flowDiagSolution()->firstAncestorOrThisOfType<RimEclipseResultCase>();
-    if ( !eclCase ) return;
+    if ( !eclCase ) return {};
 
     RigCaseCellResultsData* caseCellResultsData = eclCase->results( RiaDefines::PorosityModelType::MATRIX_MODEL );
     RigEclipseResultAddress mobPoreVolResAddr( RiaDefines::ResultCatType::STATIC_NATIVE, RiaResultNames::mobilePoreVolumeName() );
@@ -130,5 +130,5 @@ void RigFlowDiagStatCalc::mobileVolumeWeightedMean( size_t timeStepIndex, double
 
     const RigActiveCellInfo* actCellInfo = m_resultsData->activeCellInfo( m_resVarAddr );
 
-    RigWeightedMeanCalc::weightedMeanOverCells( &weights, values, nullptr, false, actCellInfo, true, &mean );
+    return RigWeightedMeanCalc::weightedMeanOverCells( &weights, values, nullptr, false, actCellInfo, true );
 }