Add support to process null strings in attributes

- Checking for sentinel value in string ID creating a null string instance.

Author: josesimoes

src/CLR/Core/TypeSystem.cpp

@@ -7183,7 +7183,16 @@ HRESULT CLR_RT_AttributeParser::ReadString(CLR_RT_HeapBlock *&value)
 
     NANOCLR_READ_UNALIGNED_UINT16(tk, m_blob);
 
-    NANOCLR_CHECK_HRESULT(CLR_RT_HeapBlock_String::CreateInstance(*value, CLR_TkFromType(TBL_Strings, tk), m_assm));
+    // check for invalid string ID: "null" string
+    if (tk == 0xFFFF)
+    {
+        // create a null string
+        NANOCLR_CHECK_HRESULT(CLR_RT_HeapBlock_String::CreateInstance(*value, nullptr, m_assm));
+    }
+    else
+    {
+        NANOCLR_CHECK_HRESULT(CLR_RT_HeapBlock_String::CreateInstance(*value, CLR_TkFromType(TBL_Strings, tk), m_assm));
+    }
 
     // need to box this
     value->PerformBoxing(desc.m_handlerCls);