Fix closing modal
Fix closing modal

Use empty string instead of null when unsetting body overflow property

Closes #4

Signed-off-by: Jesse Stuart <a5c95b3d7cb4d0ae05a15c79c79ab458dc2c8f9e@jessestuart.ca>

bellhops/react-modal,cupcake/react-modal,bellhops/react-modal,cupcake/react-modal