fix using null string in vector (#7872)

Use 0 offset as special value. 0 offset is not a valid relative offset, so it's safe to use 0 offset to indicate value is null.

#7846

Author: phenixxy

net/FlatBuffers/FlatBufferBuilder.cs

@@ -438,7 +438,8 @@ public void AddOffset(int off)
             if (off > Offset)
                 throw new ArgumentException();
 
-            off = Offset - off + sizeof(int);
+            if (off != 0)
+                off = Offset - off + sizeof(int);
             PutInt(off);
         }
 

net/FlatBuffers/Table.cs

@@ -65,7 +65,11 @@ public static int __indirect(int offset, ByteBuffer bb)
         // Create a .NET String from UTF-8 data stored inside the flatbuffer.
         public string __string(int offset)
         {
-            offset += bb.GetInt(offset);
+            int stringOffset = bb.GetInt(offset);
+            if (stringOffset == 0)
+                return null;
+
+            offset += stringOffset;
             var len = bb.GetInt(offset);
             var startPos = offset + sizeof(int);
             return bb.GetStringUTF8(startPos, len);