Add getInvalidRandomEngine() for CPU (#4528)

#4443 deleted the default constructor of RandomEngine for GPU. However
for CPU still the default constructor must be used, forcing \#ifdef to
be used in code that uses the constructor. This PR adds a
`getInvalidRandomEngine()` function to get a dummy RandomEngine. Note
the only use for RandomEngine on CPU is to make GPU code work on CPU.

Author: AlexanderSinn

Src/Base/AMReX_RandomEngine.H

@@ -68,6 +68,15 @@ namespace amrex
 
 #endif
 
+    AMREX_FORCE_INLINE
+    RandomEngine getInvalidRandomEngine () {
+#ifdef AMREX_USE_GPU
+        return RandomEngine{nullptr};
+#else
+        return RandomEngine{};
+#endif
+    }
+
 }
 
 #endif

Src/Particle/AMReX_WriteBinaryParticleData.H

@@ -67,11 +67,7 @@ fillFlags (Container<int, Allocator>& pflags, const PTile& ptile, F const& f)
     for (int k = 0; k < np; ++k) {
         const auto p = ptd.getSuperParticle(k);
         if constexpr (IsCallable<F,decltype(p),RandomEngine>::value) {
-#ifdef AMREX_USE_GPU
-            flag_ptr[k] = f(p,RandomEngine{nullptr});
-#else
-            flag_ptr[k] = f(p,RandomEngine{});
-#endif
+            flag_ptr[k] = f(p,getInvalidRandomEngine());
         } else {
             flag_ptr[k] = f(p);
         }