Fix false "SITE IS DISCONNECTED" error on empty query results

[anamwp]

Fixes #1829

Problem

wp stream query runs a connectivity check (CLI::connection()) before executing, by running a real query for 1 record and treating a falsy result as "disconnected":

$query = wp_stream_get_instance()->db->query( [ ... ] );

if ( ! $query ) {
    \WP_CLI::error( 'SITE IS DISCONNECTED' );
}

DB::get_records() returns a plain array() when the query runs successfully but matches zero rows. In PHP, an empty array is falsy, so "no records yet" and "the query actually failed" were indistinguishable — any site with an empty wp_stream table (a fresh install, or a multisite subsite with no site-level activity logged) hit SITE IS DISCONNECTED even though nothing was actually wrong.

This matches the report in #1829: one multisite install logs plenty of activity and works fine, while a second logs almost nothing (only network-level actions) and fails on every wp stream query call.

Fix

Check $wpdb->last_error instead of the query's return shape. WordPress only populates last_error when a query genuinely fails at the database level (bad SQL, missing table, dropped connection, etc.), never for a query that simply matched nothing. This restores the original intent of the check — "is the database actually reachable" — without misreading an empty-but-healthy result as a failure.

Testing

• Reproduced the bug locally: network-activated Stream on a multisite install, created a fresh subsite with zero logged activity, ran wp stream query --url=<subsite> → SITE IS DISCONNECTED (confirmed the root cause, not just multisite-specific — a single-site install with an empty wp_stream table hits the same bug).
• Applied the fix, re-ran the same commands against the root site and two empty subsites — all now return a clean, empty results table instead of erroring.
• Verified the check still fires correctly on a genuine failure: temporarily renamed away the wp_stream table and confirmed SITE IS DISCONNECTED is still reported, then restored the table.
• Full PHP test suite: 392 passing (pre-existing skipped/incomplete tests unrelated to this change).
• composer lint (PHPCS): clean.

Test_CLI coverage added for the fix (both the empty-result and genuine-failure paths):

Checklist

• Project documentation has been updated to reflect the changes in this pull request, if applicable. (N/A — internal WP-CLI logic fix, no user-facing docs affected)
• I have tested the changes in the local development environment (see contributing.md).
• I have added phpunit tests.

Release Changelog

• Fix: wp stream query no longer reports "SITE IS DISCONNECTED" when a site simply has no logged activity yet.