General: Fix code quality issues which were identified by static analysis.

This fixes minor issues that could cause PHP notices under the right conditions, and fixes some general incorrectness.

Props jrf, hellofromTonya for review

See #52217


git-svn-id: https://develop.svn.wordpress.org/trunk@51850 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/class-wp-list-table.php

@@ -207,12 +207,14 @@ class WP_List_Table {
 	 * @since 4.0.0
 	 *
 	 * @param string $name Property to check if set.
-	 * @return bool Whether the property is set.
+	 * @return bool Whether the property is a back-compat property and it is set.
 	 */
 	public function __isset( $name ) {
 		if ( in_array( $name, $this->compat_fields, true ) ) {
 			return isset( $this->$name );
 		}
+
+		return false;
 	}
 
 	/**
@@ -313,6 +315,8 @@ class WP_List_Table {
 		if ( isset( $this->_pagination_args[ $key ] ) ) {
 			return $this->_pagination_args[ $key ];
 		}
+
+		return 0;
 	}
 
 	/**