#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/ResultStatisticsCache/RigStatisticsCalculator.cpp

@@ -89,27 +89,34 @@ void RigStatisticsCalculator::addDataToHistogramCalculator( RigHistogramCalculat
 //--------------------------------------------------------------------------------------------------
 ///
 //--------------------------------------------------------------------------------------------------
-void RigStatisticsCalculator::mobileVolumeWeightedMean( size_t timeStepIndex, double& mean )
+std::optional<double> RigStatisticsCalculator::mobileVolumeWeightedMean( size_t timeStepIndex )
 {
+    return {};
 }
 
 //--------------------------------------------------------------------------------------------------
 ///
 //--------------------------------------------------------------------------------------------------
-void RigStatisticsCalculator::mobileVolumeWeightedMean( double& mean )
+std::optional<double> RigStatisticsCalculator::mobileVolumeWeightedMean()
 {
-    double sum     = 0.0;
-    size_t tsCount = timeStepCount();
+    double sum                 = 0.0;
+    size_t contributingTsCount = 0;
+
+    const size_t tsCount = timeStepCount();
     for ( size_t tIdx = 0; tIdx < tsCount; tIdx++ )
     {
-        double meanForTimeStep;
-        mobileVolumeWeightedMean( tIdx, meanForTimeStep );
-        sum += meanForTimeStep;
-    }
-    if ( tsCount != 0 )
-    {
-        mean = sum / tsCount;
+        // Time steps without a computable mean, ie. when the mobile pore volume is missing, are not
+        // allowed to contribute to the accumulated sum
+        if ( const auto meanForTimeStep = mobileVolumeWeightedMean( tIdx ) )
+        {
+            sum += *meanForTimeStep;
+            contributingTsCount++;
+        }
     }
+
+    if ( contributingTsCount == 0 ) return {};
+
+    return sum / contributingTsCount;
 }
 
 //--------------------------------------------------------------------------------------------------