Uninformative Jupyter cell magic error if the query is missing #57

Environment details

BigQuery client v1.24.0, all Python versions.

Steps to reproduce

1. Fire up a Jupyter notebook or an IPython session and load the bigquery extension:

    %load_ext google.cloud.bigquery
   

2. Use the bigquery cell magic without specifying the query:

   %%bigquery --project my-project --destination_table my_dataset.my_table
   

Actual result:
An error message is displayed, but is kind of misleading:

ERROR:
 table_id must be a fully-qualified ID in standard SQL format, e.g., "project.dataset.table_id", got 

It might make the user wonder why the table ID is incorrect even if --destination is correctly specified.

Expected result:
The error message should explain that the query is missing in the cell body.

Proposed solution

The current implementation assumes that any query without whitespace represents a table ID, but it fails to check if the query is non-empty in the first place. The latter check should be added.