fix : Allow MySQL database creation without selected database

[Driftlux]

Summary

• Allow MySQL-compatible connections to execute CREATE DATABASE / CREATE SCHEMA without a selected database.
• Keep the existing selected-database requirement for ordinary MySQL queries.
• Add focused coverage for the empty-database execution guard.

Root Cause

The query execution guard rejected every non-single-database connection when the active tab had an empty database value. On a fresh MySQL server where DBX only shows system databases and no user database is selectable, this blocks CREATE DATABASE ..., leaving users with no way to create the first database from the query editor.

Validation

• ./node_modules/.bin/vitest run apps/desktop/src/composables/__tests__/useSqlExecution.spec.ts
• ./node_modules/.bin/vue-tsc --noEmit --project apps/desktop/tsconfig.json
• ./node_modules/.bin/oxlint apps/desktop/src/composables/useSqlExecution.ts apps/desktop/src/composables/__tests__/useSqlExecution.spec.ts

[t8y2]

Thanks for the contribution! Merged in 427d0c5, will be released in the next version.