Fix throwing exception when calling RunClassConstructor on a generic …

…type with a static constructor

#99183 seems to have done away with assuming that a generic type's static constructor was
always "initialized". As a result, if you call RunClassConstructor on it, the runtime would throw an exception.

Fixes #103891

src/coreclr/vm/methodtable.cpp

@@ -3768,6 +3768,10 @@ void MethodTable::CheckRunClassInitThrowing()
     if (IsSharedByGenericInstantiations())
         return;
 
+    // For back-compat reasons, act like it has already been initialized
+    if (ContainsGenericVariables())
+        return;
+
     EnsureStaticDataAllocated();
     DoRunClassInitThrowing();
 }

src/coreclr/vm/reflectioninvocation.cpp

@@ -1306,7 +1306,8 @@ extern "C" void QCALLTYPE ReflectionInvocation_RunClassConstructor(QCall::TypeHa
         return;
 
     MethodTable *pMT = typeHnd.AsMethodTable();
-    if (pMT->IsClassInited())
+    // The ContainsGenericVariables check is to preserve back-compat where we assume the generic type is already initialized
+    if (pMT->IsClassInited() || pMT->ContainsGenericVariables())
         return;
 
     BEGIN_QCALL;