Multiple fixes for the Data connect emulator

[joehan]

Description

Fixes a number of issues causing the data connect emulator to be totally broken in 13.29.0

1 - No longer provide User and Database when starting pglite. If these are passed in and the corresponding roles/databases don't exist in dataDir/--import, the emulator will crash.

2 - The ClearData query would fail with a null value in query string of EXECUTE statement if there were no tables in the DB. This new SQL is safe in that case

3 - Actually pipe thru --debug all the way to the emulator - forgot this step in yesterdays PR 🤦

Fixes #8062 8062

Scenarios Tested

• From https://github.com/firebaseextended/codelab-dataconnect-web, start up the emulator and confirm that it does not crash.
• Next, shut down the emulator and set a dataDir. Start up again and confirm it doesn't crash.
• Next, shut down and start up again with a populated data dir. Confirm that there is no crash.
• curl -X POST localhost:4400/dataconnect/clearData/, confirm no crash.
• Add a type to your schema, add some data using VSCE, confirm it writes successfully.
• Shut down, restart, confirm data is still there.
• curl -X POST localhost:4400/dataconnect/clearData/, confirm data is cleared
• Modify schema, confirm it migrates correctly

[yuchenshi]

nit:

Suggested change

	EXECUTE COALESCE(_clear, 'select now()');
	EXECUTE COALESCE(_clear, 'SELECT now()');