Explicitly return undefined when event is undefined

[major-mayer]

Fixes #514

[hperrin]

This is now fixed here: https://github.com/hperrin/svelte-material-ui/commit/c55145231e1350eaf38b1e2ec79b17c9a0f76ca4#diff-54062c270ea651b890f69db1e1c4243b2c9033075870d648ab407cca159b67b7

That function now always returns an event, or throws an error.