Add the symbols from Sample Node

Sampling table with `POISSONIZED` (such as `SELECT * FROM table TABLESAMPLE POISSONIZED (10) RESCALED;`) would get the error `java.lang.IllegalArgumentException: Key '$sampleweight' not present in map`. This issue was also reported before in #1307.

That problem is due to that the output symbols of Sample Node are not imported. This PR adds the symbols of Sample Node and fixes this problem.
Looking at this again, I think we only need to add the sample weight symbol (if it's present)