PHP 7.4 Compatibility

- TestCase: Check if `$this->last_modifying_query` is not empty
- Database:
-- Point to the right Log class
-- Update the `getLastModifyingQuery()` return doc

src/classes/PHPUnit/Database.php

@@ -8,6 +8,7 @@
 namespace WPAcceptance\PHPUnit;
 
 use WPAcceptance\EnvironmentFactory;
+use WPAcceptance\Log;
 
 /**
  * Database trait
@@ -17,7 +18,7 @@ trait Database {
 	/**
 	 * Get the last INSERT, UPDATE, or DELETE that happened in the MySQL server
 	 *
-	 * @return array
+	 * @return array|null
 	 */
 	protected function getLastModifyingQuery() {
 		$mysql = EnvironmentFactory::get()->getMySQLClient();

src/classes/PHPUnit/TestCase.php

@@ -64,7 +64,7 @@ public function tearDown() {
 
 		$new_last_modifying_query = $this->getLastModifyingQuery();
 
-		if ( ! empty( $new_last_modifying_query ) && $new_last_modifying_query['event_time'] !== $this->last_modifying_query['event_time'] ) {
+		if ( ! empty( $this->last_modifying_query ) && ! empty( $new_last_modifying_query ) && $new_last_modifying_query['event_time'] !== $this->last_modifying_query['event_time'] ) {
 			Log::instance()->write( 'Test modified the database (' . $this->getName() . ').', 1, 'warning' );
 			Log::instance()->write( 'Last query at ' . $new_last_modifying_query['event_time'] . ': ' . $new_last_modifying_query['argument'], 2 );