Fixes #13551. Guard against illegal data access by undefined elem-owner

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>

src/data.js

@@ -85,7 +85,7 @@ Data.prototype = {
 		// Either a valid cache is found, or will be created.
 		// New caches will be created and the unlock returned,
 		// allowing direct access to the newly created
-		// empty data object.
+		// empty data object. A valid owner object must be provided.
 		var cache = this.cache[ this.key( owner ) ];
 
 		return key === undefined ?
@@ -255,9 +255,12 @@ jQuery.fn.extend({
 			var data,
 					camelKey = jQuery.camelCase( key );
 
-			// Get the Data...
-			if ( value === undefined ) {
-
+			// The calling jQuery object (element matches) is not empty
+			// (and therefore has an element appears at this[0]) and the
+			// `value` parameter was not undefined. An empty jQuery object
+			// will result in `undefined` for elem = this[0] which will
+			// throw an exception if an attempt to read a data cache is made.
+			if ( elem && value === undefined ) {
 				// Attempt to get data from the cache
 				// with the key as-is
 				data = data_user.get( elem, key );

test/unit/data.js

@@ -670,3 +670,14 @@ test( "JSON data- attributes can have newlines", function() {
 	equal( x.data("some").foo, "bar", "got a JSON data- attribute with spaces" );
 	x.remove();
 });
+
+test(".data doesn't throw when calling selection is empty. #13551", function() {
+	expect(1);
+
+	try {
+		jQuery( null ).data( "prop" );
+		ok( true, "jQuery(null).data('prop') does not throw" );
+	} catch ( e ) {
+		ok( false, e.message );
+	}
+});