Make empty strings (and other non-string values) simply return null from...

Make empty strings (and other non-string values) simply return null from parseJSON. Also added some parseJSON tests. Fixes #5859.

src/core.js

@@ -472,25 +472,24 @@ jQuery.extend({
 	},
 	
 	parseJSON: function( data ) {
+		if ( typeof data !== "string" || !data ) {
+			return null;
+		}
+		
 		// Make sure the incoming data is actual JSON
 		// Logic borrowed from http://json.org/json2.js
-		if (/^[\],:{}\s]*$/.test(data.replace(/\\(?:["\\\/bfnrt]|u[0-9a-fA-F]{4})/g, "@")
+		if ( /^[\],:{}\s]*$/.test(data.replace(/\\(?:["\\\/bfnrt]|u[0-9a-fA-F]{4})/g, "@")
 			.replace(/"[^"\\\n\r]*"|true|false|null|-?\d+(?:\.\d*)?(?:[eE][+\-]?\d+)?/g, "]")
-			.replace(/(?:^|:|,)(?:\s*\[)+/g, ""))) {
+			.replace(/(?:^|:|,)(?:\s*\[)+/g, "")) ) {
 
 			// Try to use the native JSON parser first
-			if ( window.JSON && window.JSON.parse ) {
-				data = window.JSON.parse( data );
-
-			} else {
-				data = (new Function("return " + data))();
-			}
+			return window.JSON && window.JSON.parse ?
+				window.JSON.parse( data ) :
+				(new Function("return " + data))();
 
 		} else {
 			jQuery.error( "Invalid JSON: " + data );
 		}
-		
-		return data;	
 	},
 
 	noop: function() {},

test/unit/core.js

@@ -805,3 +805,28 @@ test("jQuery.proxy", function(){
 	// Use the string shortcut
 	jQuery.proxy( thisObject, "method" )();
 });
+
+test("jQuery.parseJSON", function(){
+	expect(7);
+	
+	equals( jQuery.parseJSON(), null, "Nothing in, null out." );
+	equals( jQuery.parseJSON( null ), null, "Nothing in, null out." );
+	equals( jQuery.parseJSON( "" ), null, "Nothing in, null out." );
+	
+	same( jQuery.parseJSON("{}"), {}, "Plain object parsing." );
+	same( jQuery.parseJSON('{"test":1}'), {"test":1}, "Plain object parsing." );
+	
+	try {
+		jQuery.parseJSON("{a:1}");
+		ok( false, "Test malformed JSON string." );
+	} catch( e ) {
+		ok( true, "Test malformed JSON string." );
+	}
+	
+	try {
+		jQuery.parseJSON("{'a':1}");
+		ok( false, "Test malformed JSON string." );
+	} catch( e ) {
+		ok( true, "Test malformed JSON string." );
+	}
+});
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