bug: shouldn't be trying blank project ID when no project ID is found

I get a `NotFound` error where pandas-gbq has tried to send a query with an empty string as the project ID. This could be improved to show a better error message that the project ID was not properly detected.

```
---------------------------------------------------------------------------
NotFound Traceback (most recent call last)
[/usr/local/lib/python3.10/dist-packages/pandas_gbq/gbq.py](https://localhost:8080/#) in run_query(self, query, max_results, progress_bar_type, **kwargs)
468 logger.debug("Requesting query... ")
--> 469 query_reply = self.client.query(
470 query,

13 frames
NotFound: 404 POST https://bigquery.googleapis.com/bigquery/v2/projects//jobs?prettyPrint=false:

Error 404 (Not Found)!!1

*{margin:0;padding:0}html,code{font:15px/22px arial,sans-serif}html{background:#fff;color:#222;padding:15px}body{margin:7% auto 0;max-width:390px;min-height:180px;padding:30px 0 15px}* > body{background:url(//[www.google.com/images/errors/robot.png](http://www.google.com/images/errors/robot.png)) 100% 5px no-repeat;padding-right:205px}p{margin:11px 0 22px;overflow:hidden}ins{color:#777;text-decoration:none}a img{border:0}@media screen and (max-width:772px){body{background:none;margin-top:0;max-width:none;padding-right:0}}#logo{background:url(//[www.google.com/images/branding/googlelogo/1x/googlelogo_color_150x54dp.png](http://www.google.com/images/branding/googlelogo/1x/googlelogo_color_150x54dp.png)) no-repeat;margin-left:-5px}@media only screen and (min-resolution:192dpi){#logo{background:url(//[www.google.com/images/branding/googlelogo/2x/googlelogo_color_150x54dp.png](http://www.google.com/images/branding/googlelogo/2x/googlelogo_color_150x54dp.png)) no-repeat 0% 0%/100% 100%;-moz-border-image:url(//[www.google.com/images/branding/googlelogo/2x/googlelogo_color_150x54dp.png](http://www.google.com/images/branding/googlelogo/2x/googlelogo_color_150x54dp.png)) 0}}@media only screen and (-webkit-min-device-pixel-ratio:2){#logo{background:url(//[www.google.com/images/branding/googlelogo/2x/googlelogo_color_150x54dp.png](http://www.google.com/images/branding/googlelogo/2x/googlelogo_color_150x54dp.png)) no-repeat;-webkit-background-size:100% 100%}}#logo{display:inline-block;height:54px;width:150px}

404. That’s an error.

The requested URL /bigquery/v2/projects//jobs?prettyPrint=false was not found on this server. That’s all we know.

Location: None
Job ID: a28c0f2b-09b4-4346-97a3-0569b95b32cb

During handling of the above exception, another exception occurred:

GenericGBQException Traceback (most recent call last)
[/usr/local/lib/python3.10/dist-packages/pandas_gbq/gbq.py](https://localhost:8080/#) in process_http_error(ex)
384 raise QueryTimeout("Reason: {0}".format(ex))
385
--> 386 raise GenericGBQException("Reason: {0}".format(ex))
387
388 def download_table(

GenericGBQException: Reason: 404 POST https://bigquery.googleapis.com/bigquery/v2/projects//jobs?prettyPrint=false:

Error 404 (Not Found)!!1

*{margin:0;padding:0}html,code{font:15px/22px arial,sans-serif}html{background:#fff;color:#222;padding:15px}body{margin:7% auto 0;max-width:390px;min-height:180px;padding:30px 0 15px}* > body{background:url(//[www.google.com/images/errors/robot.png](http://www.google.com/images/errors/robot.png)) 100% 5px no-repeat;padding-right:205px}p{margin:11px 0 22px;overflow:hidden}ins{color:#777;text-decoration:none}a img{border:0}@media screen and (max-width:772px){body{background:none;margin-top:0;max-width:none;padding-right:0}}#logo{background:url(//[www.google.com/images/branding/googlelogo/1x/googlelogo_color_150x54dp.png](http://www.google.com/images/branding/googlelogo/1x/googlelogo_color_150x54dp.png)) no-repeat;margin-left:-5px}@media only screen and (min-resolution:192dpi){#logo{background:url(//[www.google.com/images/branding/googlelogo/2x/googlelogo_color_150x54dp.png](http://www.google.com/images/branding/googlelogo/2x/googlelogo_color_150x54dp.png)) no-repeat 0% 0%/100% 100%;-moz-border-image:url(//[www.google.com/images/branding/googlelogo/2x/googlelogo_color_150x54dp.png](http://www.google.com/images/branding/googlelogo/2x/googlelogo_color_150x54dp.png)) 0}}@media only screen and (-webkit-min-device-pixel-ratio:2){#logo{background:url(//[www.google.com/images/branding/googlelogo/2x/googlelogo_color_150x54dp.png](http://www.google.com/images/branding/googlelogo/2x/googlelogo_color_150x54dp.png)) no-repeat;-webkit-background-size:100% 100%}}#logo{display:inline-block;height:54px;width:150px}

404. That’s an error.

The requested URL /bigquery/v2/projects//jobs?prettyPrint=false was not found on this server. That’s all we know.

Location: None
Job ID: a28c0f2b-09b4-4346-97a3-0569b95b32cb
```