Fix for unconstrained generic types

Author: tamasvajk

csharp/extractor/Semmle.Extraction.CSharp/Entities/Expression.cs

@@ -190,7 +190,7 @@ private static bool ContainsPattern(SyntaxNode node) =>
             {
                 // = null, = default, = default(T), = new MyStruct()
                 // we're generating a default expression:
-                return Default.CreateGenerated(cx, parent, childIndex, location, parameter.Type.IsValueType ? null : ValueAsString(null));
+                return Default.CreateGenerated(cx, parent, childIndex, location, parameter.Type.IsReferenceType ? ValueAsString(null) : null);
             }
 
             // const literal:

csharp/ql/test/library-tests/parameters/Parameters.expected

@@ -45,6 +45,6 @@ withDefaultValue
 | Parameters.dll:0:0:0:0 | M7 | Parameters.dll:0:0:0:0 | e3 | 2 | Parameters.dll:0:0:0:0 | (...) ... | 0 |
 | Parameters.dll:0:0:0:0 | M7 | Parameters.dll:0:0:0:0 | e4 | 3 | Parameters.dll:0:0:0:0 | (...) ... | 1 |
 | Parameters.dll:0:0:0:0 | M7 | Parameters.dll:0:0:0:0 | e5 | 4 | Parameters.dll:0:0:0:0 | (...) ... | 5 |
-| Parameters.dll:0:0:0:0 | M8 | Parameters.dll:0:0:0:0 | t | 0 | Parameters.dll:0:0:0:0 | default | null |
+| Parameters.dll:0:0:0:0 | M8 | Parameters.dll:0:0:0:0 | t | 0 | Parameters.dll:0:0:0:0 | default | - |
 | Parameters.dll:0:0:0:0 | M9 | Parameters.dll:0:0:0:0 | t | 0 | Parameters.dll:0:0:0:0 | default | - |
 | Parameters.dll:0:0:0:0 | M10 | Parameters.dll:0:0:0:0 | t | 0 | Parameters.dll:0:0:0:0 | default | null |