Reset value to an empty string when undefined or null

[felixlheureux]

Undefined and null values are not properly handled. When an undefined value is passed in Monaco, the textModel will call factory.create on an undefined value which will throw an undefined error.

Also, if a null value is passed, since the logic returns without updating the value, the previous data will still be displayed.

A solution is to reset the value to an empty string.

Issue: microsoft/monaco-editor#3414

[alexdima]

Thanks for pointing this out. The fix should be to throw when an invalid argument is passed in.

[alexdima]

Thank you!