Fix `sqlcmd` for duckdb

[mehtamohit013]

Currently, %sqlcmd tables return none, when used in conjunction with duckdb.

Code Snippet:

import pandas as pd
df = pd.read_excel('https://go.microsoft.com/fwlink/?LinkID=521962')

%load_ext sql
%sql duckdb://
%sql --persist df

%sqlcmd tables

[edublancas]

we need to investigate more, because using the native command from DuckDB doesn't return the table:

%%sql
show tables;

(empty result)

https://duckdb.org/docs/guides/meta/list_tables.html