[PL/TH2M] Default initialize scalars to NaN.

ProcessLib/TH2M/ConstitutiveRelations/Base.h

@@ -47,8 +47,8 @@ struct MediaData
 
 struct TemperatureData
 {
-    double T;
-    double T_prev;
+    double T = nan;
+    double T_prev = nan;
 };
 
 using ReferenceTemperatureData =

ProcessLib/TH2M/ConstitutiveRelations/FluidDensity.h

@@ -19,10 +19,10 @@ namespace ConstitutiveRelations
 struct FluidDensityData
 {
     // gas phase density
-    double rho_GR = 0.;
+    double rho_GR = nan;
 
     // liquid phase density
-    double rho_LR = 0.;
+    double rho_LR = nan;
 
     static auto reflect()
     {

ProcessLib/TH2M/ConstitutiveRelations/MassMoleFractions.h

@@ -18,10 +18,10 @@ namespace ConstitutiveRelations
 {
 struct MassMoleFractionsData
 {
-    double xnCG = 0.;
-    double xmCG = 0.;
-    double xnWL = 0.;
-    double xmWL = 0.;
+    double xnCG = nan;
+    double xmCG = nan;
+    double xnWL = nan;
+    double xmWL = nan;
 
     static auto reflect()
     {

ProcessLib/TH2M/ConstitutiveRelations/SolidThermalExpansion.h

@@ -19,8 +19,8 @@ template <int DisplacementDim>
 struct SolidThermalExpansionData
 {
     KelvinVector<DisplacementDim> solid_linear_thermal_expansivity_vector;
-    double beta_T_SR;  /// Isotropic solid phase volumetric thermal expansion
-                       /// coefficient.
+    double beta_T_SR = nan;  /// Isotropic solid phase volumetric thermal
+                             /// expansion coefficient.
     double thermal_volume_strain = nan;
 };