Revert "fix using null string in vector (#7872)" (#8879)

This reverts commit 1cb1c4baee.

net/FlatBuffers/FlatBufferBuilder.cs

@@ -438,7 +438,6 @@ namespace Google.FlatBuffers
             if (off > Offset)
                 throw new ArgumentException();
 
-            if (off != 0)
             off = Offset - off + sizeof(int);
             PutInt(off);
         }

net/FlatBuffers/Table.cs

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