Fixed issue calling DataSnapshot methods with null data

[rhodgkins]

Description

Fixes firebase/firebase-functions-test#254

Fix is similar to the check in #exists():

firebase-functions/src/common/providers/database.ts

Line 173 in 5f7b331

if (typeof val === "undefined" || val === null) {

[inlined]

@taeold IIUC this is a straight bugfix, but I wanted to double check that you don't consider this a breaking change.

[taeold]

Feels like this is a good change and not breaking.

IIUC the change would only break if the traversing the full path instead of short-circuiting somehow had a side effect that was worthy.