don't unset paging state if null ptr passed

The code of `cass_statement_set_paging_state_token` was unsetting the
paging state on a statement if a null paging state pointer was passed.
This is inconsistent with the behavior of other setters, which preserve
the existing state if a null pointer is passed. The code is changed
to preserve the existing paging state in this case.

Note: CPP Driver does not check the passed pointer for null at all.

Author: wprzytula

scylla-rust-wrapper/src/statement.rs

@@ -422,7 +422,6 @@ pub unsafe extern "C" fn cass_statement_set_paging_state_token(
     };
 
     if paging_state.is_null() {
-        statement_from_raw.paging_state = PagingState::start();
         return CassError::CASS_ERROR_LIB_NULL_VALUE;
     }