Add defensive coding to posts per page sniff and add tests

The posts per page sniff should bail out early if empty string is passed as a value.

The tests were added for both posts per page sniff and slow db query sniff,
to check if empty string is passed as a value.
In the case of SlowDBQuery the sniff should flag cases where there is and isn't a value passed,
as that sniff will always flag whenever meta_key and meta_value are used in a query.

WordPress/Sniffs/DB/SlowDBQuerySniff.php

@@ -9,6 +9,7 @@
 
 namespace WordPressCS\WordPress\Sniffs\DB;
 
+use PHPCSUtils\Utils\TextStrings;
 use WordPressCS\WordPress\AbstractArrayAssignmentRestrictionsSniff;
 
 /**

WordPress/Sniffs/WP/PostsPerPageSniff.php

@@ -67,6 +67,10 @@ public function getGroups() {
 	public function callback( $key, $val, $line, $group ) {
 		$stripped_val = TextStrings::stripQuotes( $val );
 
+		if ( '' === $stripped_val ) {
+			return false;
+		}
+
 		if ( $val !== $stripped_val ) {
 			// The value was a text string. For text strings, we only accept purely numeric values.
 			if ( preg_match( '`^[0-9]+$`', $stripped_val ) !== 1 ) {

WordPress/Tests/DB/SlowDBQueryUnitTest.inc

@@ -21,3 +21,7 @@ $query = 'foo=bar&meta_key=foo&meta_value=bar';
 if ( ! isset( $widget['params'][0] ) ) {
 	$widget['params'][0] = array();
 }
+
+$query = 'foo=bar&meta_key=&meta_value=bar';
+$query = 'foo=bar&meta_key=foo&meta_value=';
+$query = 'foo=bar&meta_key=&meta_value=';

WordPress/Tests/DB/SlowDBQueryUnitTest.php

@@ -44,6 +44,9 @@ public function getWarningList() {
 			15 => 1,
 			16 => 1,
 			19 => 2,
+			25 => 2,
+			26 => 2,
+			27 => 2,
 		);
 	}
 }

WordPress/Tests/WP/PostsPerPageUnitTest.inc

@@ -124,3 +124,17 @@ $args = array(
 	'posts_per_page' => 75.0, // OK (75).
 	'posts_per_page' => 150.000, // Bad (150).
 );
+
+$query = 'posts_per_page=' . (int) $_POST['limit']; // OK.
+
+$args = array(
+	'posts_per_page' => '', // OK.
+);
+
+_query_posts( 'nopaging=true&posts_per_page=' ); // OK.
+
+$query_args['posts_per_page'] = ''; // OK.
+
+$query_args[
+	'posts_per_page'
+] = ''; // OK.