fixes bug with two-way bindings trying to set input empty string valu…

…e to an observable that doesn't exist yet

can-stache-key-test.js

@@ -260,3 +260,9 @@ testHelpers.dev.devOnlyTest("a warning is given for `callMethodsOnObservables: t
 	QUnit.equal(teardown(), 1, "warning displayed");
 });
 
+QUnit.test("writing to a null observable is ignored", function(){
+	observeReader.write({},"foo.bar", "value");
+	observeReader.write(null,"bar", "value");
+	observeReader.write(null,"foo.bar", "value");
+	QUnit.ok(true, "all passed without error");
+});

can-stache-key.js

@@ -294,14 +294,16 @@ observeReader = {
 		var last;
 
 		options = options || {};
-
 		if(keys.length > 1) {
 			last = keys.pop();
 			parent = observeReader.read(parent, keys, options).value;
 			keys.push(last);
 		} else {
 			last = keys[0];
 		}
+		if(!parent) {
+			return;
+		}
 		var keyValue = peek(parent, last.key);
 		// here's where we need to figure out the best way to write