Core: Fixed error when calling .rules() on empty jquery set.

Closes #1706.

src/core.js

@@ -117,6 +117,12 @@ $.extend( $.fn, {
 
 	// http://jqueryvalidation.org/rules/
 	rules: function( command, argument ) {
+
+		// If nothing is selected, return nothing; can't chain anyway
+		if ( !this.length ) {
+			return;
+		}
+
 		var element = this[ 0 ],
 			settings, staticRules, existingRules, data, param, filtered;