Fix crash caused by passing null as constant value when creating Literal bound node

src/Compilers/Core/Portable/Compilation/Expression.cs

@@ -46,7 +46,7 @@ public static ConstantValue SynthesizeNumeric(ITypeSymbol type, int value)
                 return SynthesizeNumeric(((INamedTypeSymbol)type).EnumUnderlyingType, value);
             }
 
-            return null;
+            return ConstantValue.Bad;
         }
 
         public static BinaryOperationKind DeriveAdditionKind(ITypeSymbol type)